Failover is the automatic switching of a router from its primary connection to a backup connection when the primary fails. The intent is to keep a site online with minimal disruption: the device, the camera, or the payment terminal continues working, even though the path back to the network has changed.

In short: Common failover patterns include fixed-line broadband with cellular backup (a retail store keeps its tills running if the fibre cuts out), dual-SIM cellular (two SIMs from different operators in one router, switching if one fails), and dual-modem cellular (some Teltonika routers, like the RUTX12 and RUTM52, keep two cellular connections active simultaneously for hot failover with no reconnection delay). Detection of failure is based on link state, ping tests, or a combination of both.

How long failover takes matters. A simple link-state failover might take 30 seconds to detect a failure and switch. Hot failover with two active links can be sub-second. For a website, 30 seconds is fine. For a payment terminal mid-transaction, it is not. For mission-critical CCTV or industrial control, dual-modem hot failover or carrier-diverse dual-SIM is usually the right answer.

Failover only works if the backup is actually independent of the primary. A backup SIM on the same network as the primary will not help if that network is having a regional outage. Carrier-diverse failover (two SIMs on different MNOs, or multi-network SIMs that can roam between operators) protects against operator-level failures, which are statistically more common than individual cell-tower failures.