Trade-In Cancelled/ According to tracking number Assurant picked up trade-in and has apparently lost it. Help.

I received an email that my trade in was cancelled with no other explanation. According to the USPS tracking number, the phone was delivered and picked up by an individual in Lewisville, TX on October 26th. (It was due on October 31.) 

I contacted Xfinity Mobile via chat who told me that I had to contact Assurant. I contacted Assurant and was told that I had to contact Xfinity Mobile. I am now stuck in a frustrating "customer service" loop and the unhelpful Xfinity Agent AI has no idea how to handle a question about a lost trade-in. 

I need to know how to get this issue resolved as my $1000 trade-in phone is currently lost in the ether after being picked up by Assurant. 

Thank you, in advance, for your assistance with this extremely frustrating issue.