Returned device never received by AT&T

I recently ordered a new iPhone 11 Pro Max on my account, but while the phone was in transit to me, I got a different phone and cell-phone plan from my new job. So, I decided not to keep the phone, and returned the device back to AT&T using the shipping label provided in the box. I never even opened the iPhone box!

   Before mailing it back to AT&T, I even inquired in the local AT&T store if I could drop it off, but they said they don't take returns in the store for the phones ordered online.

   I dropped off the phone at a local PostalAnnexe (along with another Amazon return package). I assumed everything was fine, and I even got a confirmation from Amazon that they received the Amazon return < 4 days later.

   AT&T continued to charge me for the phone even after 3 weeks of returns, and I openeed a case with them inquiring about the returned phone. 1.5 months, 2 cases, and 10 phone calls later, they still do not have an answer. The only thing they say is that I should have retained the tracking number when I mailed it in, and with out that, they cannot help! They even have failed to give me any details or information about the shipping label they provided me. I was asking them which warehouse the package would have been sent, or any tracking information on the label, but they have no clue! One of the reps even had accused me not not mailing in the phone because they didnt receive it!

   I have had it with AT&T! This is a $1200 phone, and they have utterly no responsibility in helping track the package. Every time I call, they ask me to call about a week later, and this is now 1.5 months now! Meanwhile, they are charged the monthly payment twice already, despite the fact that the phone IMEI is not registered (and they have no idea how to locate it!)