iPhone 7 A1778 128gb not compatible with T-mobile data network

Does anyone know why an iPhone 7 A1778 / model MN9H2LL/A (128 gb) will not activate on the 4G LTE data network of a T-Mobile MVNO but a different iPhone 7 A1778 / model MN9D2LL/A (32 gb) will activate and operate fine. Both are SIM unlocked from AT&T, both place and receive calls fine, and are current on iOS 15.7.4. I have restarted the 128 gb phone multiple times, reset it completely, reset just the network settings, reinserted the SIM card multiple times. Tech support for the MVNO claims that the phone is only "partially" compatible with their network lacking support for either band 4 or band 12. Based on the specs for the phone that is a very incorrect statement. Originally I wondered if it was part of the Intel/Qualcomm modem issue with this phone but according to the information I found the A1778 only had Intel modems and the A1660 had the Qualcomm. Any help is appreciated.