I know you can't tether with your phone or a hotspot device on the Unlimited Data Plan but can you use the AT&T Beam USB modem as an extra device?
The reason I ask is you could buy a Surface 3 4G LTE tablet and add that as a tablet device to the unlimited plan.
I have a Surface 3 (Wifi Only). Adding the modem would make it function the same as the tablet with 4g.