I just switched on 5 lines. 3 were setup at ATT and are connecting no problem. 2 of the phones, Samsung GS5 and LG G3, have no data. They are both unlocked T Mobile phones. What setting do I need to change?
Have you checked that all the server data and credentials have been set up properly? Usually it works without issues by just inserting the SIM (as long as the phone is compatible with AT&T's frequencies), but you might need the carrier to send you the right configuration. I believe this can be done via SMS or they can guide you step by step over the phone.