iPhone previously unlocked by AT&T but now showing as carrier locked
I previously helped my mom her originally AT&T locked Phone 6 Plus unlocked; it worked fine, she used Mint Mobile for a while before getting a newer phone. She gave us the original for our daughter. I was able to activate the phone with Apple, but upon trying to insert a SIM I'm unable to activate. I called Apple and they say that they are showing as carrier locked to A&T.
I tried to submit a new unlock request but receiving the message "We can't unlock this device for you because AT&T did not sell or activate it."
Any help is greatly appreciated!