The RUT901 Dual SIM Router provides the user with a single CAT4 4G Modem for cellular connection to the Internet.  It also has two sim card slots, earning it the name of dual SIM router.  

This means you can insert SIM cards from two separate networks and configure the router to use each one in particular circumstances.

For example, SIM 1 may be an unlimited data SIM with a fair use policy cap of 500GB per month - the second SIM can be a backup so if the data limit is reached, and SIM 1 provider restricts access to the Internet, the router can switch to the backup SIM card in SIM slot 2 and the router can continue to provide cellular Internet connectivity, switching back to SIM 1 on a particular date (for example the first day of the month) when the SIM 1 service is available again.

DUAL SIM does not mean that you have have two connections working at the same time, nor does it mean you can aggregate or bond connections.  The router only has a single 4G modem, so can only establish a single 4G connection using EITHER SIM 1 OR SIM 2.

How Dual SIM Failover Actually Works on the RUT901

The RUT901 runs Teltonika's RutOS firmware, which gives you a lot of control over exactly when and why the router switches between SIM 1 and SIM 2. This is not a simple toggle. You configure trigger conditions, and the router acts on them automatically.

The switching triggers available in RutOS include: weak signal (RSSI drops below a threshold you set), data limit reached, no network available, network denied, data connection failed, roaming detected, and SIM idle protection. You can combine these. For example, switch to SIM 2 if signal drops below -95 dBm OR if the data connection has failed for 60 seconds.

SIM idle protection is worth mentioning separately. Some mobile networks will deactivate a SIM that sits unused for too long. The RUT901 can periodically activate the backup SIM just to keep it registered on the network, then switch back. This means your backup SIM is actually ready when you need it, not dormant and potentially deactivated.

You also get signal monitoring in real time - RSSI, RSRP, RSRQ, SINR - so you can see exactly why a switch happened and whether your thresholds are set correctly.

Failover is not instant. There is a registration delay when switching SIMs, typically 15-60 seconds depending on the network. If your application cannot tolerate any downtime at all, dual SIM alone is not the answer - you need a router with two separate modems, or a different architecture entirely.

Do You Actually Need Two SIM Cards?

Possibly not.

The reason most people fit a backup SIM is network coverage gaps or data cap management. A multi-network roaming SIM solves the coverage problem without needing a second slot. These SIMs carry profiles for multiple UK networks - typically EE, Vodafone, O2 and Three - and connect to whichever has the strongest signal at that location. If EE drops out, the SIM roams to Vodafone. The router doesn't need to do anything. There is no switching delay, no configuration, no idle protection issue to worry about.

For most fixed-site IoT deployments - ANPR cameras, kiosks, vending machines, remote monitoring - a single multi-network SIM in a single-SIM router is more reliable than a dual SIM setup using two single-network SIMs.

The eSIM Angle

Some Teltonika routers now support eSIM alongside a physical SIM slot. The more significant development is SGP.32 - the IoT eSIM standard that allows remote SIM provisioning over the air. A router with built-in SGP.32 eSIM support can download and switch operator profiles remotely, without anyone physically touching the device.

This changes the calculation for dual SIM considerably. If you can reprovision the eSIM to a different operator remotely - because coverage has changed, because a better commercial deal is available, or because an operator has withdrawn service at that site - you no longer need a physical backup SIM sitting in slot 2 doing nothing for months.

The Teltonika RUTX routers and newer TRB series gateways are adding eSIM support. For new deployments, it is worth asking whether a single-SIM-plus-eSIM router gives you more flexibility than a traditional dual SIM setup - because in most cases, it does.

Summary

The RUT901 is a solid choice if you have two specific SIM cards you want to manage with rule-based switching. RutOS gives you granular control over when and why it switches. If your primary concern is coverage rather than SIM management, a multi-network roaming SIM may make the dual SIM feature largely redundant. And if you are specifying new hardware for a long-term deployment, look at eSIM-capable routers - the ability to switch operators remotely without touching the device is a significant operational advantage.