Behold the Netgear 4G LTE Modem with Two Ethernet Ports. You plug this little fella between your existing Internet connection and your router, and if your DSL or Cable Modem goes out, this takes care of automatic failover to Cellular LTE.
It works with any US-based GSM-based carrier, which is everybody except Sprint and their affiliates. You stick in the SIM card. I bought a Mint Mobile Test Kit to see if it would work with them. It does.
I will either stick with them or switch to FreedomPop for $44/year. It’s only 1GB/month but that’s why this is for failover.
For configuration, I needed to look up the network name for Mint Mobile, it was something like “MINTMOBILE.” It will of course vary with your choice of carrier.
On the house end of this, we have it plugged directly into our eero Mesh WiFi system. It’s the first time I can remember actually getting whole-house WiFi – not without trying other solutions (remember DD-WRT???).