Straight from
Apple.com
Cellular and Wireless
Model A1633*
Model A1634*
LTE (Bands 1, 2, 3, 4, 5, 7, 8, 12, 13, 17, 18, 19, 20, 25, 26, 27, 28, 29, 30)
TD-LTE (Bands 38, 39, 40, 41)
TD-SCDMA 1900 (F), 2000 (A)
UMTS/HSPA+/DC-HSDPA (850, 900, 1700/2100, 1900, 2100 MHz)
CDMA EV-DO Rev. A (800, 1700/2100, 1900, 2100 MHz)
GSM/EDGE (850, 900, 1800, 1900 MHz)
Model A1688*
Model A1687*
LTE (Bands 1, 2, 3, 4, 5, 7, 8, 12, 13, 17, 18, 19, 20, 25, 26, 27, 28, 29)
TD-LTE (Bands 38, 39, 40, 41)
TD-SCDMA 1900 (F), 2000 (A)
CDMA EV-DO Rev. A (800, 1700/2100, 1900, 2100 MHz)
UMTS/HSPA+/DC-HSDPA (850, 900, 1700/2100, 1900, 2100 MHz)
GSM/EDGE (850, 900, 1800, 1900 MHz)
All models
802.11a/b/g/n/ac Wi‑Fi with MIMO
Bluetooth 4.2 wireless technology
NFC
Looks like the only difference on the AT&T models are the extra LTE 30 band. There should be no issue using the phone on T-Mobile other than T-Mobile's Network performance and coverage will vary depending on where you live and travel.