AT&T Charging me for a returned iPhone XS MAX

In August I purchased an iPhoneXSMAX and then returned it in exchange for an iPhoneXS.

I shipped it with the AT&T return label from the USPS in manhattan and have been continuing to be charged for the device ever since.

- The USPS confirmed from the tracking number that a AT&T hired pick up received the device and that AT&T is liable for any loss beyond that point

- AT&T has on 2 occasions confirmed to me on the phone that the iPhone was in their warehouse, once they tried to tell me the reason they would give me NO refund was because headphones were missing, which is not true.

​

What do I do if AT&T refuses to acknowledge that they owe me a return? I have called them over 2 dozen times and each time they promise to resolve it and they promise it will be reflected on my next bill and then I call again and they tell me that they don't owe me anything... I am beyond frustrated and now with coronavirus I am pinched and really can't afford an additional $50 monthly bill that I should not owe.