Samsung S5 not recognizing T-mobile sim card after update

falconfund

    I have a G900A Samsung galaxy S5, the AT&T one, unlocked running 4.4.4 android. It was working fine with the tmobile sim card I had in, then I flashed the same version (G900A) with an updated ROM that has android 5.1.1 (an official distribution). The sim card is not recognized now, all attempts to go into network settings are met with something along the lines of "please insert sim card to access network settings."

    In Device Information, it says carrier, imei, and all that are "unknown"

    What can I do to fix this?

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