No credit for trade in phone

I purchased the new iPhone intending to trade in my old iPhone 8+ for a credit. I received a padded shipping envelope from AT&T with an address in La Vergne, TN. I packaged my phone and shipped it mid December and the USPS update on the tracking number says it arrived at the Nashville, TN Distribution Center Annex on December 24th, but will be "arriving late". It has said this for the last 4 weeks. 

I have been reading about issues with this address not being open anymore or being a scam, which AT&T should know before sending out return envelopes. 

I have not been able to get ahold of anyone on their customer service lines who can help and I'm about to file a complaint with the BBB and my state's Attorney General as I've seen others do. The credit should be applied to my account and all charges for the new phone removed immediately.