If it's a sizable operation where a little downtime is costly, it can be worth setting up a cheap prepaid LTE hotspot as a backup. Probably the best way is to plug a LTE stick into a Raspberry Pi or similar, then configure it as the backup router. Set up the switches in a chain with the routers on opposite ends (ring/mesh if they support STP), thus minimizing the number of single failure points.
https://wiki.openwrt.org/doc/recipes/high-availability