Stupid question regarding wireless hotspots

So I’m gonna give a TL;DR at the end but for those who want to bother I have a buddy who has a 45 foot boat within a couple minutes of my house (he lives two hours away) and I have been helping him use a AT&T hotspot for wireless internet in order for these temp stick sensors he has to send back temperature data to his phone via internet. All worked well for awhile but then his old hotspot just randomly died so he went to AT&T and got a new one. Ever since the temp sticks will connect to the hotspot but will only stay that way for 30 minutes to a hour before disconnecting until someone resets them.

My thought is maybe this Unite Express 2 hotspot they gave him is a piece of junk and there are prolly some nice hotspots out there that are better than what AT&T gives (I know it’ll be more expensive) and will work properly assuming AT&T lets you put one of their SIM cards in a device you got from another source

Both the temp stick company and AT&T say this setup should work without issue so it’s not like they can’t work through a hotspot

TL;DR: can I buy a hotspot online then use it with AT&T’s LTE network?