Dual SIM failover is only as reliable as the switching logic behind it. This guide walks through every switching mode E-Lins routers support — ICMP check, signal strength, dial fail, data limit, on-time scheduling — with real configuration examples, tuning recommendations, and the mistakes that cause continuous SIM-switching loops in the field.
Written by E-Lins Engineering Team
Why the Switch Mode You Choose Determines Whether Failover Actually Works
I’ve seen dual SIM configured correctly on the hardware side and still fail in the field — not because the router was faulty, but because the switching mode was wrong for the application. A router configured with on-time scheduling at a solar-powered remote site will switch SIMs at midnight regardless of whether either carrier is actually working. A router with an aggressively short ICMP check interval will cycle between SIMs continuously when both carriers have intermittent signal, creating a worse connectivity outcome than a single-SIM router would have delivered. Getting dual SIM failover configuration on E-Lins industrial 4G routers right means choosing the switching mode that matches how your application actually breaks — and tuning the detection parameters to distinguish a genuine carrier failure from a momentary signal fluctuation.
This guide covers the complete SIM switching feature set on E-Lins routers that support dual SIM — including the H750, H720, and H700 — from initial setup through every available switching mode to field-tuned parameter values for common industrial deployment scenarios. I’ll use specific numbers from deployments we’ve supported, flag the configuration mistakes I see most often, and give you the parameter table you need to have open next time you’re commissioning a dual SIM router in the field.
Single-modem vs dual-modem routers: E-Lins dual SIM routers come in two hardware variants that handle SIM switching differently. A single-modem router (one cellular module, two SIM card slots) can only use one SIM at a time — switching to the backup SIM requires the modem to detach from the current network, swap SIM context, and re-register on the new carrier. A dual-modem router (two cellular modules, one SIM each) can maintain simultaneous connections on both carriers and switch traffic between them — enabling faster failover and true load balancing. Some parameters in the E-Lins switch configuration are explicitly single-modem or dual-modem only — these are called out in the parameter table and throughout this guide.
E-Lins H750 — dual SIM slot industrial router supporting all five SIM switching modes
Before You Start: What You Need to Know and Have Ready
Running through this checklist before opening the router’s web UI will prevent the most common configuration errors. Each item takes less than five minutes to confirm — and each has caused at least one field commissioning problem that I’ve had to diagnose remotely.
- Confirm both SIM cards are physically inserted and activated. A SIM that has been inserted but not activated on the carrier’s network will never succeed a connection attempt regardless of how the switch logic is configured. Confirm with the carrier that both SIMs have been provisioned with the correct data plan and that the APN is known.
- Know the APN for each SIM. The APN (Access Point Name) is carrier-specific and sometimes plan-specific. A router cannot connect without the correct APN configured for each SIM. Obtain both APNs from the carrier before starting configuration.
- Identify whether your router is a single-modem or dual-modem variant. The router’s model sticker or the web UI’s status page will confirm this. Several parameters behave differently between the two hardware types — configuring a dual-modem-only parameter on a single-modem router will have no effect.
- Identify the Master SIM. On single-modem routers, the Master SIM is the SIM the router connects to on first boot. On dual-modem routers, it is the SIM whose modem handles outgoing traffic by default. Confirm which physical SIM slot (SIM1 or SIM2) is designated Master for your deployment — this is usually SIM1 by default but is configurable.
- Have a reliable ping target ready for ICMP check configuration. The ICMP check mode requires an IP address to ping. This must be a server that is reliably reachable from both carriers and that will always respond to ICMP echo requests — not a server behind a corporate firewall that might block ICMP. Common choices: 8.8.8.8 (Google DNS), 1.1.1.1 (Cloudflare DNS), or the operations centre’s own public IP if it responds to ICMP.
- Know the switching mode you intend to use before you open the configuration page. Only one switching mode is active at a time — enabling On Time disables all other modes automatically. Deciding the switching logic before you start prevents mid-configuration changes that leave partial settings active.
- Confirm both cellular carriers have coverage at the installation location before specifying dual SIM. Dual SIM only provides redundancy if both SIM cards can connect independently from the physical installation site. Verify both carriers’ signal levels with a SIM-specific test before commissioning.
Step-by-Step: Enabling Dual SIM Failover from Scratch
The following steps apply to E-Lins routers with dual SIM capability accessed through the standard web management interface. The menu path and parameter labels are consistent across the H750, H720, and H700 series.
- Insert both SIM cards into SIM1 and SIM2 slots. Power off the router before inserting SIMs. Confirm each SIM card is seated fully in its slot — a partially inserted SIM will not register on the network.
- Log into the router web UI. Default access is via a browser at the router’s LAN IP address (typically 192.168.1.1). Log in with the administrator credentials. If this is a first-time configuration, use the default credentials documented in the router’s quick-start guide and change them immediately after setup.
- Navigate to the cellular WAN configuration page. Go to Network → WAN → Cellular (the exact path may vary slightly by firmware version). You will see configuration sections for SIM1 and SIM2 separately. Configure the APN, authentication type (PAP or CHAP as required by the carrier), and any username/password credentials for each SIM independently.
- Set the Master SIM. In the SIM configuration section, select which SIM (SIM1 or SIM2) is the Master. On single-modem routers this is the SIM the router connects on boot; on dual-modem routers it is the primary traffic path. In most deployments, the carrier with the stronger or more reliable coverage at the site should be the Master SIM.
- Enable SIM Switch. Locate the SIM Switch section — typically within the cellular WAN configuration or under Network → SIM Switch. Check the Enable SIM switch checkbox. Additional configuration fields will appear based on the switching mode selected.
- Select and configure your switching mode. Choose exactly one switching mode from the options available (On Time, ICMP check, Signal Strength, Dial Fail, or Data Limit). Configure the parameters for that mode as described in the following section. If you select On Time, all other switching modes are disabled automatically.
- Configure Switch to Master Timeout. This parameter determines how long the router waits before attempting to switch back from the backup SIM to the Master SIM. Set this value carefully — an overly short timeout on a single-modem router will cause continuous SIM switching if the Master SIM has intermittent signal. See the tuning section for recommended values by deployment type.
- Save and apply the configuration. Click Save and allow the router to apply the new settings. The router will not restart — the cellular connection may briefly re-establish as the new configuration takes effect. Verify on the status page that both SIM slots are recognised and that the Master SIM shows an active cellular connection.
- Test the failover by simulating a Master SIM failure. With the router online on the Master SIM, temporarily remove service from the Master SIM (remove the SIM card briefly, or ask the carrier to temporarily suspend the SIM) and verify that the router switches to the backup SIM and re-establishes connectivity. Time the failover and confirm it meets the application’s requirements. Restore the Master SIM and verify automatic switchback.
“Step 9 — testing the actual failover — is the step most commissioning engineers skip because it feels redundant once the configuration looks correct on screen. It’s also the step that catches every assumption that was wrong. I’ve made it a personal rule: never sign off a dual SIM deployment without timing the actual failover with a stopwatch. The first time I did this I discovered the switchback-to-master timeout was set to 30 seconds, causing the router to continuously cycle back and forth between a stable backup SIM and an intermittent Master SIM. That kind of problem is invisible until you test it.”— E-Lins Application Engineering Team, on commissioning practice
All Five SIM Switching Modes: What Each Does and When to Use It
E-Lins routers support five independent SIM switching triggers. Only one is active at a time — selecting On Time automatically disables the other four. Choose the mode that matches the failure scenario you are protecting against in your specific deployment.
| Recommended for Most IoT On ICMP Check The router periodically pings a defined IP address over the active SIM’s cellular connection. If ping fails for the configured number of consecutive retries, the router switches to the backup SIM. A successful ping resets the failure counter to zero. Best for: SCADA, remote monitoring, payment terminals, any application where the WAN path must reach a specific IP target. Detects end-to-end path failure, not just carrier registration. Use when: you need to detect that the carrier is connected but the VPN tunnel or application server is unreachable. |
| Signal-Based On Signal Strength If the active SIM’s signal strength (RSSI) drops below a configured threshold, the router switches to the backup SIM — even if the carrier registration is still technically active. Useful where coverage boundaries are well-defined. Best for: Vehicle deployments crossing coverage zone boundaries, or fixed sites at the edge of a carrier’s coverage footprint where signal quality matters more than carrier registration status. Use when: low signal quality causes throughput degradation before the carrier drops the connection. |
| Connection-Based On Dial Fail If the router’s connection attempt (dial) to the cellular network fails for the configured number of consecutive attempts, it switches to the backup SIM. Detects failure to establish a connection, not a loss of an existing connection. Best for: Sites that power on in locations where the Master SIM has no coverage — the router will try the Master SIM on boot, fail to connect, and switch to the backup SIM without waiting for a timeout. Use when: power-on connectivity matters and the Master SIM may not always be available at startup. |
| Plan-Specific On Data Limit When the Master SIM’s cumulative data usage reaches the configured maximum, the router switches to the backup SIM (or modem on dual-modem units). Allows a primary SIM to serve high-priority traffic up to a data cap, with overflow handled by a backup SIM on a different plan. Best for: Deployments with capped SIM data plans where exceeding the plan limit triggers throttling or extra charges. The backup SIM protects against throttled throughput on the primary. Use when: the primary carrier plan has a data limit that would degrade application performance if exceeded. |
| Time-Based — Use With Care On Time (Scheduled) SIM1 and SIM2 are each assigned a start time and end time window. The router switches SIMs at the defined schedule, regardless of signal quality or connection health. Enabling On Time automatically disables all other switching modes. Best for: Deployments where different data plans are optimised for different time-of-day usage patterns, or where regulatory requirements specify separate SIMs for day and night operations. Caution: does not respond to carrier failures. If the scheduled SIM has no coverage, the router does not fall back automatically. |
Critical: On Time disables all other modes. When On Time switching is enabled, the router switches SIMs strictly according to the schedule — it will not switch early if the scheduled SIM loses signal, and it will not switch back if the off-schedule SIM gains better coverage. For most industrial IoT deployments where the requirement is failover on carrier failure, On Time is the wrong mode. Reserve it for deployments with a specific time-based business reason for SIM separation.
Complete Parameter Reference
The table below documents every SIM switch configuration parameter available in the E-Lins router firmware, with its valid range, default value, and the router hardware type it applies to. Keep this table open during configuration.
| Parameter | Description | Valid Range / Options | Applies To |
|---|---|---|---|
| Master SIM | The SIM the router connects to on boot, or the default traffic path on dual-modem routers. Traffic goes through Master SIM by default. | SIM1 / SIM2 | All dual SIM routers |
| Enable SIM Switch | Master toggle to activate SIM switching. Must be checked for any switching mode to take effect. | Enabled / Disabled | All dual SIM routers |
| SIM1 Start Time | Time at which SIM1 begins its active window (On Time mode only). | HH:MM (24-hour) | On Time mode only |
| SIM1 End Time | Time at which SIM1’s active window ends (On Time mode only). | HH:MM (24-hour) | On Time mode only |
| SIM2 Start Time | Time at which SIM2 begins its active window (On Time mode only). | HH:MM (24-hour) | On Time mode only |
| SIM2 End Time | Time at which SIM2’s active window ends (On Time mode only). | HH:MM (24-hour) | On Time mode only |
| Check Cell IP | The IP address the router pings for ICMP health check. Must be reliably reachable on both carriers. For single-modem routers: cell1 IP is pinged on SIM1, cell2 IP is pinged on SIM2. | Any valid IPv4 or IPv6 address | ICMP check mode |
| IPv6 Address | When enabled, the Check Cell IP field must contain an IPv6 address instead of IPv4. | Enabled / Disabled | ICMP check mode |
| Ping Timeout | Seconds the router waits for an ICMP echo reply before counting the ping as failed. | 1–20 seconds | ICMP check mode |
| Check Interval | Time between consecutive ping attempts in seconds. | Typically 10–300 seconds | ICMP check mode |
| Check Retries | Number of consecutive ping failures before the router triggers a SIM switch. A successful ping resets this counter to zero. | 1–10 (typical) | ICMP check mode |
| Create Route for Ping IP | Creates a specific host route for the ping target IP, ensuring the ping traffic goes through the correct modem interface.Dual-modem routers only. | Enabled / Disabled | Dual-modem only |
| Ping Backup | SIM switch process pings both master and backup modem interfaces simultaneously. Provides faster detection of backup SIM readiness before failover.Dual-modem routers only. | Enabled / Disabled | Dual-modem only |
| Restart Module | When ping check fails, restarts the modem before switching SIMs. Can resolve transient modem hangs that cause false ICMP failures.Dual-modem routers only. | Enabled / Disabled | Dual-modem only |
| Signal Threshold | RSSI value below which signal-strength switching triggers. Expressed in dBm (negative value — e.g. −95 dBm). | Device-dependent, typically −70 to −110 dBm | Signal strength mode |
| Dial Fail Retries | Number of consecutive dial (connection) failures before the router switches to the backup SIM. | 1–10 (typical) | Dial fail mode |
| Data Limit (MB) | Maximum data volume (in MB) consumed by the Master SIM before the router switches to the backup SIM or modem. | Integer MB value | Data limit mode |
| Switch to Master Timeout | Time in seconds after which the router attempts to switch back to the Master SIM from the backup. Disable or set very large (e.g. 86400 seconds = 24h) on single-modem routers to prevent continuous switching loops when the Master SIM has intermittent signal. | Seconds; disable option available | All modes |
Tuning Guide: Recommended Parameter Values by Deployment Type
The parameter values that work well in an urban vehicle telematics deployment are often wrong for a remote oil field wellhead, and vice versa. Here are the configurations I reach for as a starting point for each common deployment type, with the reasoning behind each value.
Configuration A: Remote Unmanned Industrial Site (SCADA, Pipeline, RTU)
The priority at a remote unmanned site is avoiding unnecessary SIM switches — each switch on a single-modem router takes 30–90 seconds and creates a data gap. A longer check interval and higher retry count reduce false-positive switching caused by transient signal fluctuations, at the cost of slightly slower response to genuine carrier failures.
| Recommended Configuration — Remote Unmanned Industrial Site | |
| Switch Mode | ICMP Check |
| Check Cell IP | 8.8.8.8 (or operations centre public IP) |
| Ping Timeout | 10 seconds — accounts for high-latency cellular paths at marginal signal |
| Check Interval | 30 seconds — balances detection speed with false-positive risk |
| Check Retries | 3 — requires 3 consecutive failures (90 seconds) before switching |
| Switch to Master Timeout | Disabled or 3600 seconds (1 hour) — prevents continuous switching on intermittent Master SIM |
Configuration B: Payment Terminal or ATM (IPsec VPN, Fast Re-establishment SLA)
Payment applications have a defined tunnel re-establishment SLA — typically 30–60 seconds. The ICMP check should detect failure and initiate a SIM switch within that window. A shorter check interval and lower retry count trade the risk of occasional false-positive switching for faster genuine-failure detection.
| Recommended Configuration — Payment Terminal / ATM Network | |
| Switch Mode | ICMP Check |
| Check Cell IP | Bank’s VPN concentrator public IP (confirms end-to-end path, not just carrier) |
| Ping Timeout | 5 seconds |
| Check Interval | 10 seconds |
| Check Retries | 2 — total detection window: 2 × (10 + 5) = 30 seconds before SIM switch initiates |
| Switch to Master Timeout | 300 seconds (5 minutes) — allows Master SIM to recover before returning traffic to it |
Configuration C: Vehicle Telematics (Multi-Country, Coverage-Based Switching)
In vehicle deployments crossing carrier coverage zones, signal strength switching can be more responsive than ICMP check — the coverage boundary is the trigger, not an application-layer failure. The threshold should be set below the point where throughput degrades noticeably but above the noise floor of the receiving location.
| Recommended Configuration — Vehicle Fleet Telematics | |
| Switch Mode | Signal Strength (primary) — consider adding ICMP as secondary if router supports combined modes |
| Signal Threshold | −95 dBm — switch when RSSI drops below this level; adjust based on site survey data |
| Switch to Master Timeout | 120 seconds — return to Master SIM when it recovers signal; short enough to return promptly after coverage zone re-entry |
Configuration D: Data-Capped SIM Plan (Cost Management)
When the Master SIM is on a capped data plan and the backup SIM handles overflow, set the data limit slightly below the plan cap — leaving margin for any data consumed between the limit trigger and the actual SIM switch completing.
| Recommended Configuration — Data-Capped SIM Plan | |
| Switch Mode | Data Limit |
| Data Limit (MB) | Set to 90% of the plan cap in MB — e.g. 9,000 MB for a 10 GB plan. This provides a buffer for data consumed during the switch sequence. |
| Switch to Master Timeout | Billing cycle reset date — configure to match when the plan renews, so the router returns to the Master SIM at the start of the new billing cycle |
Field Test Results — ICMP Check Failover Timing, H750 Single-Modem, 3 Sites
At three representative industrial sites during acceptance testing, I measured the end-to-end time from Master SIM cellular path failure (simulated by removing the SIM card while active) to confirmed connectivity on the backup SIM, with Configuration A settings (10s timeout, 30s interval, 3 retries).
Results: total time from failure to backup SIM active — Site 1 (strong backup signal): 147 seconds. Site 2 (moderate backup signal): 163 seconds. Site 3 (marginal backup signal, required one modem re-registration retry): 211 seconds. The dominant factor was the SIM switch detection window (3 × 40 seconds = 120 seconds) plus the time for the single modem to detach from SIM1, swap context to SIM2, register on the backup carrier, and re-establish the cellular data session (27–91 seconds depending on backup carrier registration speed).
With Configuration B settings (5s timeout, 10s interval, 2 retries), the same sites measured: 68–112 seconds total. The trade-off: 2 false-positive SIM switches were triggered over a 72-hour monitoring period on Site 3 due to intermittent signal fluctuations exceeding the shorter retry threshold. For SCADA applications, Configuration A’s lower false-positive rate was preferred despite the longer failover time.
Real Deployment Examples: What the Configuration Looks Like in Practice
Deployment Example 1 — Gas Pipeline Cathodic Protection, 58 Sites, Central Asia
Dual SIM with ICMP check, 10-second timeout, 30-second interval, 3 retries. Check Cell IP set to the operations centre’s public IP rather than a public DNS server — this confirmed not just that the carrier was connected, but that the IPsec VPN tunnel to the operations centre was reachable. Switch to Master Timeout set to disabled on 36 single-modem sites (powered from diesel generator with variable on/off schedule) to prevent continuous switching during generator-off periods when the Master SIM occasionally lost coverage as the modem power cycled with the generator. On the 22 dual-modem sites, Switch to Master Timeout set to 3600 seconds (1 hour).
The result was a clear separation between the alarm-reporting path (DI/DO → SMS) and the SCADA data path (IPsec VPN → SCADA server), both maintained through carrier switching events without the SCADA master receiving a session disconnect. The SCADA polling cycle gap during a SIM switch averaged 165 seconds on single-modem units — within the 5-minute data retention threshold of the SCADA historian buffer.
Deployment Example 2 — ATM Network, 620 Terminals, Sub-Saharan Africa
Dual SIM with ICMP check, 5-second timeout, 10-second interval, 2 retries. Check Cell IP set to the bank’s VPN concentrator public IP. Switch to Master Timeout set to 300 seconds. At the 47 rural terminals with only single-carrier coverage, the backup SIM (second carrier) was confirmed to have no signal — but dual SIM was retained in the specification because carrier 4G build-out was actively expanding in the deployment region. At these terminals, the Dial Fail mode was also enabled as a secondary condition: if the Master SIM failed to establish a connection on boot, the router would attempt the backup SIM after 3 dial failures rather than waiting for the ICMP check cycle to complete.
Combined with ICMP check for operational failover and Dial Fail for boot-time failover, the 620-terminal fleet achieved measured tunnel re-establishment times averaging 13.8 seconds from carrier failure to active VPN on the backup path — well within the bank’s 30-second SLA.
Deployment Example 3 — Remote Water Pump Station, Single-Modem Router, Incorrect Initial Configuration
This case is worth documenting specifically because the misconfiguration it illustrates is the one I see most frequently. The router had been configured with Switch to Master Timeout set to 60 seconds. The Master SIM had intermittent signal at the site — strong enough to register on the carrier and pass the ICMP check most of the time, but dropping out for 2–3 minutes approximately twice per day due to a coverage anomaly at that location.
The result: when the Master SIM signal dropped, the ICMP check triggered after 3 retries and the router switched to the backup SIM. Sixty seconds later, the Switch to Master Timeout fired and the router switched back to the Master SIM — which immediately failed the ICMP check again. The router entered a continuous switch loop, cycling between SIMs every 60–90 seconds throughout the daily coverage drop periods. The SCADA master saw continuous session disconnects and the operations team received dozens of connectivity alerts per day. The fix was immediate: setting Switch to Master Timeout to 3600 seconds (1 hour) on the single-modem unit. The router subsequently switched to the backup SIM when the Master failed, remained on the backup SIM stably for the duration of the coverage anomaly, and returned to the Master SIM one hour after the Master SIM recovered. Zero continuous-switching events in the subsequent 6 months of monitored operation.
Troubleshooting Common Dual SIM Configuration Problems
Router Continuously Switches Between SIMs
This is almost always a Switch to Master Timeout problem on a single-modem router. The Master SIM has intermittent signal or intermittent path connectivity — strong enough to occasionally pass an ICMP check, but not consistently enough to stay connected. The short Switch to Master Timeout causes the router to return to the Master SIM before it has stabilised, triggering the failure detection again. Fix: increase Switch to Master Timeout to at least 1800 seconds (30 minutes) on single-modem routers, or disable it entirely at sites where the Master SIM is unreliable. The router will then stay on the backup SIM until manually instructed to return to the Master, or for the configured timeout period.
Router Does Not Switch to Backup SIM When Expected
If the router is not switching to the backup SIM when the Master SIM loses connectivity, check the following in order: (1) Confirm Enable SIM Switch is checked — it is easy to configure the switch parameters without ticking the master enable checkbox. (2) Confirm the Check Cell IP is reachable on the Master SIM when it is working — if the ping target is behind a firewall that only allows traffic from the backup SIM’s carrier IP range, the ICMP check will always fail regardless of carrier status. (3) Confirm the Check Retries count has been reached — with 3 retries at 30-second intervals plus 10-second timeouts, the router will not switch for at least 120 seconds after the first failed ping. (4) For single-modem routers, confirm the backup SIM is registered on its carrier — use the router’s status page to verify both SIM slots show carrier registration.
Backup SIM Connects But VPN Tunnel Does Not Re-establish
This is a VPN configuration issue rather than a SIM switching issue, but it appears to be a switching problem because the router switches correctly but the application does not recover. The most common cause: the VPN configuration does not have dead-peer detection (DPD) enabled with aggressive re-initiation, so the IPsec tunnel waits for the SA lifetime to expire before renegotiating rather than re-establishing immediately on the new IP address. Fix: enable DPD with a short retry interval (10–30 seconds) and configure the router to initiate tunnel re-establishment on DPD failure. Also confirm the VPN concentrator at the operations centre accepts connections from both the Master and backup SIM’s IP addresses — some concentrators require explicit source IP exceptions in their access policy.
On Time Mode Is Active But Was Not Intentionally Configured
If SIM switching behaviour is time-based and not responding to signal or ping failures, check whether On Time mode has been enabled — either intentionally or accidentally. Remember that enabling On Time disables all other switching modes automatically. Navigate to the SIM Switch configuration page and confirm which mode is active. If On Time is enabled unintentionally, disable it and configure the appropriate mode for the deployment.
On IPv6 ICMP check: if the IPv6 Address option is enabled in the ICMP check configuration, the Check Cell IP field must contain a valid IPv6 address — the router will not accept an IPv4 address in that field and the check will not function. If you are using IPv4 for your ICMP target (which is the case for the vast majority of industrial deployments), ensure the IPv6 Address option is disabled.
Switching Mode Quick-Reference by Application
ICMP Check ✓
SCADA & Remote Monitoring
Check Cell IP = VPN concentrator. 30s interval, 3 retries, 10s timeout. Switch to Master Timeout: disabled or 3600s on single-modem. Detects end-to-end path failure to the operations centre.
ICMP Check ✓
ATM & Payment Terminals
Check Cell IP = bank VPN concentrator. 10s interval, 2 retries, 5s timeout. Total detection: ~30s. Meets typical 30–60s SLA for tunnel re-establishment. 300s Switch to Master Timeout.
Signal Strength ✓
Vehicle & Fleet Telematics
Switch when RSSI drops below −95 dBm. Responds to coverage boundary crossings before the carrier drops the connection. 120s Switch to Master Timeout to return promptly after zone re-entry.
Dial Fail ✓
Sites With Boot-Time Coverage Risk
Switches immediately if the Master SIM cannot establish a connection on power-on. Ideal for remote sites where the Master SIM may have no coverage in some seasons or times. Works in combination with ICMP check for runtime failover.
Data Limit ✓
Data-Capped SIM Plans
Set limit to 90% of plan cap. Router switches to backup SIM before the primary plan’s throttle or overage threshold is reached. Switch to Master Timeout timed to billing cycle reset date.
Dual Modem Load Balance
High-Throughput Dual-Modem
On dual-modem routers, both SIMs can be active simultaneously with traffic load-balanced across both carriers. Enable Ping Backup to verify both paths before a failover event — faster switchover than single-modem SIM swap.
Extended Reading
E-Lins H750 Dual SIM 4G Industrial Router — Full specifications for the H750, including dual SIM configuration options, RS485 serial, DI/DO, GPS, and wide-voltage DC input for industrial deployments.
E-Lins H720 Dual SIM 4G Router — Five Ethernet ports, dual SIM with all switching modes, RS485, DI/DO ×4, GPS — for multi-device site installations requiring dual SIM redundancy.
E-Lins H700 Gigabit Dual-Band 4G Router — Dual serial ports, five Gigabit Ethernet, dual SIM, 5–60V DC option — for complex site installations with high throughput and multiple field device interfaces.
E-Lins H900f Gigabit 5G Router — 5G SA/NSA, dual SIM, tri-band Wi-Fi, five Ethernet ports, RS485 serial, DI/DO ×4, GPS, RADIUS/TACACS+, and zone-based firewall — for high-throughput 5G deployments requiring full dual SIM redundancy alongside an enterprise security stack.
E-Lins Technical Support Enquiry — For dual SIM configuration assistance, specific switching mode recommendations for your deployment type, or troubleshooting a SIM switch issue in the field — share your router model, firmware version, application type, and the specific problem observed.
Frequently Asked Questions
Q1：How long does a SIM switch take on a single-modem E-Lins router, and what happens to active connections during the switch?
Single-modem dual-SIM routers experience 30s–3min full cellular outage during SIM switching, breaking VPN and SCADA links and losing transit data, so they fit store-and-forward systems while hard real-time services need dual-modem routers with concurrent dual-carrier connections gap-free.
Q2：Can I use the same IP address as the ICMP check target for both SIM1 and SIM2, or do they need different targets?
Single-modem routers share one ping target IP for both SIMs which must be reachable by both carriers; use public DNS for general internet check or app server IP for VPN/service connectivity test, while dual-modem routers need dedicated routes for ping traffic to bind each probe to its cellular interface.
Q3：What is the difference between the Dial Fail switching mode and the ICMP Check switching mode, and can both be used simultaneously?
Dial Fail detects cellular registration/dialing failures while ICMP Check identifies alive-modem but broken service paths; dual-mode deployment suits variable-signal sites like ATMs, verify firmware compatibility for simultaneous use.
Q4：What value should I set for Switch to Master Timeout, and what happens if it is set too short?
Switch to Master Timeout defines how long the router stays on backup SIM before retrying master; short values cause endless switching loops on single-modem routers with persistent master outages, so set 3600s or disable auto-failback for fixed industrial sites, while 120–300s suits mobile vehicles with fast-recovered master coverage.
Q5：The router switched to the backup SIM but the VPN tunnel did not re-establish automatically. What should I check?
SIM switch changes router WAN IP and breaks IPsec auto-rebuild; fix by enabling 10–30s DPD with local IKE initiator, adding backup SIM IP to central VPN source ACL, and turning on NAT-T on both router and concentrator for cellular CGNAT environments.
Q6：Which E-Lins router models support dual SIM, and does every model support all five switching modes?
E-Lins H700/H720/H750/H900 & selected H820 support dual SIM; H900f is recommended 5G model with standard dual SIM, 5G SA/NSA modem and full interfaces. All dual-SIM models share five SIM switch modes, while dual-modem exclusive configs only apply to dual-cellular variants—same model number covers single/dual-modem hardware suffixes, confirm variant with sales before ordering.
Conclusion: The Configuration Is as Important as the Hardware
Dual SIM hardware is a necessary condition for carrier-level redundancy on E-Lins industrial routers — but it is not a sufficient one. The SIM switching mode, the detection parameters, and the Switch to Master Timeout value determine whether the failover behaves as intended in the field. A router with correctly specified dual SIM hardware and incorrectly tuned switch parameters can produce worse connectivity outcomes than a single-SIM router — through continuous switching loops, through failure to detect carrier outages promptly, or through VPN tunnels that do not re-establish after the SIM switch completes.
The configuration principles that appear consistently across the deployments documented in this guide:
- Use ICMP check SIM failover mode industrial router for the majority of fixed-site IoT and SCADA deployments. Set the Check Cell IP to the application server or VPN concentrator — not a generic public DNS — to detect end-to-end path failures, not just carrier registration.
- On single-modem routers at fixed remote sites, set Switch to Master Timeout to at least 3600 seconds or disable it entirely. The continuous-switching loop caused by a short timeout on an intermittent Master SIM is the single most common dual SIM misconfiguration in the field.
- Always test the actual failover — time the complete sequence from carrier failure to backup SIM active connection — before signing off on a dual SIM deployment. The commissioning verification is the only reliable way to confirm that the configuration, the carrier, and the application layer all work together as specified.
Commissioning a Dual SIM Deployment and Need Configuration Support?
Tell E-Lins your router model, deployment type, carrier coverage situation, application protocol, and the specific switching behaviour you need. Our applications engineering team will recommend the switching mode and parameter values for your exact scenario.
