AT&T Unlocked Phone on T-Mobile Network? Possible issues? SM-N960U

So I bought a Note 9 listing off swappa. This listing: https://swappa.com/listing/view/LUJK37602

SM-N960U he said unlocked bought it through T Mobile. I just opened and turned it on and to my surprise it has the ATT firmware. I am very livid because when someone says "bought through...

AT&T Unlocked Phone on T-Mobile Network? Possible issues? SM-N960U