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I have a router which have both incoming internet through Ethernet and 4G via USB. My fiber connection is unstable, and I have gotten a courtesy 4G SIM from my ISP. Is it possible to set up the router in such a way that when fiber connection fails, it automatically switches to 4G?
I have an ASUS 2900 router and an open 4G USB stick.
Have you tried reading the manual for Asus RT-AC2900? A quick search for failover gives:
4.3.2 Dual WAN
Your ASUS wireless router provides dual WAN support. You can set the
dual WAN feature to any of these two modes:
Failover Mode: Select this mode to use the secondary WAN as the backup network access.
Load Balance Mode: Select this mode to optimize bandwidth, minimize response time and prevent data overload for both primary and secondary
WAN connections.
The failover mode seems like the feature you are looking for.