T-Mobile wearable plan only works with T-mo versions of Gear S and S2 because it looks up of those known IMEIs. However, if you can find a T-Mo CSR that knows what he is doing, he can enter your AT&T watch's IMEI number into the database and it will work. But this is YMMV. Some get it work. Others not so lucky.
Otherwise, if you are willing to pay full smartphone plan price, it should work as long as you get it unlocked.