Carrier SIM lock

I bought a phone Pixel 6 through ebay tied to the operator. There is a SIM card of the operator itself, but I do not know the number. I have no connection with the seller, I tried to unlock it in an official way through the AT&T's site, but it doesn’t work, I can’t contact the operator about separation, I live abroad, I don’t know what to do. What to do in this situation, who can advise?