New Asus router - easiest backup cellular option?

I just upgraded our old Netgear router to an Asus RT-AC68U. Man, this thing’s range and speed is nuts compared to what we had. I noticed it would support a USB cellular modem, or an Android as a wifi hotspot. It also has a failover dual WAN option. This is all super new to me, but I’m looking to have a cheap option that switches to a cellular signal if our internet is down.

We have an ADT SmartThings hub, which would have a backup option, but it didn’t do what I needed it to, and essentially makes me pay for a bunch of other stuff that I don’t need.

I’ve read some other threads that people have used a service called FreedomPop, but it seems like billing and service is kind of iffy.

Also, would there be a way to restrict only my ADT hub to use the cellular backup if the main internet goes down? I have a bunch of Wyze cams that are on the same wifi, that I assume would eat up data like crazy.

It seems like the Asus routers only support certain USB stick modems. Can I simply buy one that’s listed to work, go to a Walmart and buy a prepaid data sim card, stick that in the modem, and be done with it?

Thanks for the help!