Do you know the person you sold it to or was it sold on eBay or Swappa? Could it be there's some sort of scam going on?
So is it saying it is network locked when the person you sold it to puts in a T-Mobile SIM card? If so, this needs to be unlocked by AT&T. If they processed the unlock again and it is still not working, it will need to be escalated through them. There is nothing we can do at T-Mobile for a device sold by AT&T.
Hey, did you still need some help? Please check out the posts above and let us know if we can assist. Thanks.