My main aim is for when the main DSL router dies / shaped or otherwise link down. I don't mind if this requires me to login to the router and flip over from the ethernet WAN link to Wifi WAN link.
Would this be an easier approach (wifi connection... aaaand we're done) than all the permutations of USB devices and AT commands for 3g/4g modems... ? Just a thought. Nearly every 3g/4g dongle provides a Wifi hotspot, as do nearly all mobile phones these days. (I don't mean to oversimplify the permutations of wifi radios in routers

PS: Gargoyle is great - provides everything I want it to! Trying to get back to Gargoyle-USB-controlled relays; just got a more immediate need for reliable internet & redundancy.