TMobile charged me $370 for a router I returned in perfect condition and got store receipt!

In october, my neighborhood finally got fiber optic internet. As a result I went to my local store in Lynchburg, Virginia and returned the Gateway router, ARC KVD21, and cancelled the service. The store provided me with a restocking receipt. Now, in December, TMobile sends me a bill for $370+ for non return of the gateway.

I'm absolutely fuming at the incompetency of TMobile. This is extremely poor customer service. Hopefully I can get this resolved tomorrow by going to the store. I don't know if other people have been affected by this. Any advice would be appreciated.

Finally, the call center is usesless as I don't have a TMobile number to receive a 6 digit pin as I cancelled the account!