T-Mobile non return fee on gareway scam

T-Mobile is doing the same thing to me. I returned the gateway through UPS after the store refused to take it. My December bill was $0.00 but they sent a bill in January for $395.99 for an unreturned gateway. I had saved all communications from T-Mobile since April 2022 when I cancelled my service because they never cancelled my account. Unfortunately, I disposed of them when the December bill came so I no longer have the UPS tracking information. The guy at the UPS store said this is happening to a lot of people. It's interesting to me that most of these complaints are within the same timeframe of September/October through December 2022. Either someone at T-Mobile is stealing the returned gateways or there is an entire section of the warehouse they haven't checked. Also, the charges aren't consistent.