I have three routerboards with built in LTE.

One has a Tmobile Sim card and works fine. The APN is "fast.t-mobile.com"

One has a Verizon sim card and works fine, The APN is "vzwinternet"

One has a ATT Prepaid sim card with 10 gigs ($50) on the account according to the at&t account login page.I have confirmed that my iemi is registered to the sim card, and they are both associated to this account.

The APN "phone", "NXTGENPHONE", and "Broadband" all get a IP address, but limited connectivity. I can't ping , but I can do ns lookup against the assigned DNS and the browser on the connected PC gets redirected to
http://redirect.ac.sl.attcompute.com/cg ... .cgi?msg=6
It says "AT&T Free Msg: It looks like you want to use data but have none available. You may need to add a data package or change to a plan that supports data for your device. To check your plan and manage your account, login to att.com/myprepaid.

This is with AT&T in many devices.
I have oposite situation, in R11e-LTE6 the broadband and NXTGENPHONE give me internet and works from fist use.
LTE module Telit LM940 give me the same problem what you have now.

I not found solution, please give feedback if you found it. Calling to AT&T not give me any clue.

https://forums.att.com give 410K answers at this sentence :(