Didn’t not get my trade in credit for 7 months and still waiting for my $800 credit

In April 2022 I purchased an iPhone 13 Pro from @ATT online. I was ordering a new phone because there was an offer that said that I would receive $800 back in monthly statement credits in exchange for me sending AT&T my previous Android OnePlus 9 and moving from T- Mobile to AT&T. This statement credit was supposed to be $23 per month. I waited for @ATT to send my return shipping label but it didn’t come. In May 2022, I called AT&T and they then emailed me a shipping label to use for sending the old phone back. I shipped my Android phone to AT&T via FedEx using the label they provided. After 3 months of not getting my statement credit, I called AT&T and the representative said they did not receive my phone;  but there was an misunderstanding/error on their side and they would begin to issue the credits the next month.  I, in fact, did not receive any statement credits after that and called AT&T an additional two more times without getting traction on this issue. I was on those phone calls almost 2 hours each time. 
It was after the fourth time I called @ATT , when the representative said if I submitted proof of delivery they would begin to issue my statement credits. During that call I texted a copy of my shipping record to the phone number provided by the representative. The representative then said my phone was only worth a $15 credit. 
I am beyond frustrated and disappointed with @ATT and need help getting my $800 statement credit as promised with the offer. 
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