Advice needed for AT&T 100GB/$55 prepaid plan

[green73]

I'm a total cellular noob trying to setup internet for my home network. It looks like the new 100GB for $55 prepaid AT&T plan looks like the best option for me unless I go with a shady reseller (are there are any truly legit resellers offering 300GB plans?). I know I can get AT&T here; verizon may work here too, but I don't have a verizon phone to test with.

First off, I've read that normally you have to activate this plan in a AT&T store. I don't have access to a local store, but I have read a couple posts where people have been able to activate this plan online using a new prepaid sim card like what walmart or target sell. Has anyone here done that?

Second, what happens once you use up the 100GB? Can I just order another 100GB for $55 or do I have to wait till the end of the month? If the latter, can I have multiple sim cards / accounts and just swap the sim cards as needed?

Finally, what router/modem would work well with this plan? I currently have a generic (intel nuc style) pfsense box as my main router but no cellular modem. Could a simple usb modem work with pfsense? Or do I need a dedicated router for cellular.