As far as I can ascertain, using a mobile hotspot with this unlimited plan appears to be exploiting a loophole? Ebay sellers apparently set these accounts up using info meant for a tablet device. Once the account is activated by AT&T, the SIM also then works in their hotspots. At least for now? As the user account was meant for a tablet?
There is no mention on AT&T website of an unlimited 4G LTE $29.95 monthly plan for hotspots. Only for tablets - with automatic monthly billing.
Another question is whether Ebay is shutting seller accounts down who offer this plan? Notice that all these sellers have relatively few ratings?
