@ATTHelp Returned Phone for trade in issues

@ATTHelp

I returned my old S8 phone for a trade in using the return packaging material provided by ATT thas was included with my new S20 phone that I received in the mail. ATT states their was no trade in started. I  returned the phone in August under Tradeinoffer20 and I have not been given credit and no one at ATT can find my old phone. Please help. I purchased the phone at Target while ATT reps were there Cydcor Retail promoting me the free upgrade. I am being charged for the new phone every month and I was told it would be free. Please help.