Carrier lock

is there a way to remove AT&T lock on my phone so it can only be used on AT&T networks??? I’m trying to use my iPhone 12pro with a different carrier that att and it won’t read the sun. I purchased the phone thru Apple unlocked originally but when att delivered the phone they had locked it and I need this fixed., however att messed my bill up royal and wants me to pay $500 in bills I already paid but they failed to credit so my account is not bad standing and disconnected. Any help would be appreciated.