unlock AT&T phone that still owes a balance

Bought an iPhone 13 pro on a local classifieds website, the device took my t-mobile sim card and showed service. Only after when I went home did I realize that it is locked under an AT&T sim card as there is still a balance/installment due on the AT&T account. Reached out to the seller, but no response... Is there a way to unlock the phone for t-mobile without dealing with the seller? I saw some "unlock websites" but they all look like scams to me.