Trade in credit cancelled and returned to me without explanation

Hey there!

I had recently purchased an iPhone 15 PM using the trade in offer for an iPhone 12 PM of $850 on September 15th as a new customer. I received the iPhone 15 PM on release day and sent off my trade in device within the required timeframe. I am currently in the third billing cycle, one of which I was provided a trade in credit and one I was not. 

On November 14th, I received an email without explanation that stated that my trade in credit had been canceled, and if I had questions to call the customer support line. I called, and the phone agent stated that everything was fine with the credit and the email must have been a mistake. She stated that the credits should have been applied and said she would issue a credit manually. 

Then today, I received a package that contained my old iPhone 12 PM. I won't spend too much time on the packaging inside the box (there was none, my old phone was loosely in the box with a few stickers on the back), but there was no documentation whatsoever. I still literally have no idea why the trade in was canceled. 

These things being said, I feel that I should be able to return the iPhone 15 PM however am being told that I'm outside my return window. This is frustrating, as XM or Assurant held on to my device for several weeks without any indication of issue. Is there anything that can be done here? I only opted for this very expensive piece of equipment due to the trade in offer that was provided initially. 

Thanks in advance!