OK. Here’s the reason I ask: AT&T has plans (such as I showed above) which are intended for tablets, etc. Sometimes they work great in routers; sometimes not at all. And, AT&T has started to “crack down” on these as use in routers is a violation of their terms of service. We have one we use for testing in Peplink routers and another in use for 4G back-up only and both work perfectly. However, I know of two folks who were contacted by AT&T and told they’d not be renewed (I think that was the wording) and another person who has found it works “sometimes.” (The ones we have are “unlimited” and the most similar plans are now capped at 25gB/month.)
These were/are sold by a variety of sellers, including on EBay. Sometimes the seller provided a “special” IMEI" that would pass AT&T’s automated test.
So, when someone indicates they have poor/unreliable service with AT&T the first question that comes to mind is “exactly what plan do you have?” While I am not suggesting that is your issue it seems the question still begs an answer.
/set rant on The “4G situation” in the USA is a total mess – very expensive, silly rules and ToS, difficulty in provisioning, usually no competent customer support, etc. Sometimes it’s difficult to bore through all of that in order to determine if the problem is hardware, firmware or is really just a goofy problem with cellular. /set rant off