I LOST my T-Mobile and Sprint accounts simply by restoring the iPad. For some reason, the embedded sim doesn't remember your account info once you restore.
This happened two different times when I restored the iPad, both times, I lost my Sprint and T-Mobile accounts on the iPad. Literally. T-Mobile and Sprint did not know how to reactivate the iPad or how to even look it up in their system.
Apparently, once you restore the iPad, the embedded sim is given a new number so this is why any accounts associated with the iPad before restoring are then wiped out once you do restore.
It could be different with AT&T though. I haven't tried AT&T on my iPad because as you may or may not know, the embedded sim would then be locked to AT&T and to change to a different carrier, I would need Apple to change out the embedded sim.