I've implemented failover on a Mikrotik HAP that had two broadband connections and I achieved that with scripting etc. Remember it well :-)
However, I'm pretty sure that I plugged a USB LTE modem into the USB port of a basic system and it just sort of worked, i.e. if WAN on ether1 failed, it switched over to LTE. But I've just tried that on a new system and there is no automatic failover.
Is my memory playing tricks and you always have to do *some* configuration changes to get LTE failover to work on a basic reset router?