4G failover option when no Internet from the ISP?

I have all setup on my LAN, router is LinkSys WRT3200ACM, running latest Home Assistant on RP3 with SSL / DuckDNS service, accessing it from the Internet and so on, UPS making sure no power outages, … BUT… when the Internet from my ISP goes down then I get the panic attack!

So I was thinking for some 4G (LTE) failover device to plug maybe between the cable and router and to allow 4G connection only when no internet from my ISP. I am just not sure for the best way and device (brand) to go for and how to set it up. Because charges will apply when on 4G I need it really connects only when ISP Internet is down.

Is anyone familiar or has experience with such setup? And how about utilizing Google FI Sim cards for such device? 10$ per 1 GB of data.

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The Ubiquiti Edgerouter X is cheap as chips and can do dual WAN failover/failback. Setup is easy using a wizard.

Asuswrt-merlin also supports dual WAN failover/failback if you can pick up a cheap 2nd hand supported ASUS router. This also supports some USB 4G modems.

I use the Netgear LB2120 4G modem (not so cheap) with a $9 a month 1GB data plan. More than enough for occasional main WAN outages.

One thing to be aware of is that all 4G services use CGNAT. This means when on the 4G service you will not be able to log into HA remotely using DuckDNS. You will still get push messages though.

I think you should be able to use a VPN to log in but I have not tried it.

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