Can't Unlock My Phone

I need to unlock my phone. It is an AT&T phone (locked to AT&T) using Tracfone service. 

When I request an unlock, the site says, “device is on an installment plan and you haven’t paid it off yet,” but I’m not on an installment plan; I’m not even using AT&T wireless service.

My service is Tracfone (on an AT&T locked device), but Tracfone is moving from AT&T towers to Verizon towers. I was sent a new SIM card by Tracfone (which uses Verizon towers), but I can’t use the SIM card because my phone is locked to AT&T.

When I call AT&T customer service on a different phone, the robot tells me to call 611 on my usual phone, which doesn’t work because Tracfone shut off my old SIM card because they are transferring to Verizon. (I need to unlock it for my new SIM card to work, so I can’t make calls or texts).

I've talked to AT&T customer service via the online chat three times. They told me Tracfone needs to unlock my device. When I talk to Tracfone customer service, they tell me AT&T needs to unlock my device (I believe Tracfone).

Is there anything else I could do? Who's fault is this?