Dual SIM Routers enable the user to insert a single SIM card for use as the primary 4G connection and the router has a second SIM card slot to enable a second SIM Card to be inserted and the router can switch to this backup SIM card in the event that the primary SIM card / network becomes unavailable. Many Industrial 4G routers have this dual SIM failover feature to provide an extra layer of network connectivity resilience.

A Dual SIM 4G router will not be able to use both SIM cards at the same time. If you need a 4G router that can have two simultaneous connections, either for load balancing or hot swap failover then you will need a dual modem 4G router.