I think know whats going on. In your *#0011# Service Menu. When it switches to Band 2 the MCC-MNC change. It shows something like 310 260 when its on T-Mobile but 310 410 when its on AT&T. When on Band 2 for some reason it will swap from 260 on Band 4 to 410 on Band 2 therefore identifying it as an AT&T network. Why is it trying to connect to AT&T? I have no idea.
Because I havent had any issues I havent really tried to test my connection enough to know when im band 2 or band 4 or what exact mcc-mnc changes are occuring... for those worried about messing up their phones they need to just make sure they are always flashing a rom built foe their carriers' phone model and also make sure that each flash performed doesnt affect the modem.. (I know the modem part is hard due to roms being based off newer releases but as long as you are following my first suggestion (verify you are flashing the right carriers' phone model download) then there should never be an issue
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