LOST iPhone 12 VIA USPS. UNABLE TO GET UPGRADE AND STILL BEING CHARGED

I purchased an iphone 12 Pro on 11/4/2020, which was delivered 12/16/2020. I picked it up from the AT&T store on 12/16/20 and mailed it back on 12/18/20 due to dissatisfaction with the phone. I mailed the phone back via USPS  On 12/19/20 the shippign carrier: Pitney Bowes (Newgistics) picked it up at 3:14 PM. Since then the phone has been MIA and not returned to the warehouse. Because of this I am unable to undo my upgrade and reorder a new phone. I have also been charged for the last 2 months for this phone that I no longer have in my possession. I have called SEVERELY to AT&T customer service, loyalty service with no assistance. I have been given the run around with no solution. I am a physician working on the front lines during COVID who was also interested in First Net. However, nobody has helped me nor tried to make my transition a smooth process. It is completely unacceptable to be paying for a service and phone that I do not even possess. I need assistance immediately. I am highly disappointed and appalled at the lack of  promptness and assistance AT&T has provided me.