The package has not been received by ATT

Hi Team,

I returned the devices received from ATT by putting the prepaid return service label. I have the receipt that shows the tracking no. from USPS. The package has not been received by ATT, it been 3 months already. Please help resolve this issue and track the package.

I have traded in my iPhone XSmax for iPhone 12. I returned the iPhone xs max on Nov 5th, 2020.