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If the failover is to be done between two different cellular routers, you would need to implement a mechanism, such as VRRP for both devices to test their links to the outside and make a decision on which should be the "path" (master/backup)
If you want to do failover with dual SIM on each units, we also have mechanisms for testing and falling back (note however that you can only have one SIM card active at a time since there is only one cellular module inside such devices)
You can find several type of failover scenarios in the documentation section of on our support web site here: https://www.digi.com/support/productdetail?pid=5644&type=documentation
Digi Technical Support Engineer