i’m being charged a non-return fee of $220 for a router that I returned and I have proof of returning

So frustrated. In 2022. A salesman at T-Mobile talked me into adding the Internet router device that T-Mobile offers. It did not work out well for me. I kept it a short time and then I called and canceled and told them I’d like to send it back. T Mobile sent me a mailing label to print through email. it goes back through UPS. I still have a copy of it. I have the tracking number it was returned. It was received months ago on and I’m wondering my why my cell phone bill is higher than I expected it to be and I should’ve caught on right away, but I didn’t. I was still being charged for the service of the router that I canceled and returned . I called and complain and now I make sure I’m canceled ,a little time goes by and all of a sudden I receive a text message telling me that I’m being charged a non-return fee for the router! The same router that I returned several months ago. $220! I called customer service I’m on the phone over an hour. I get nowhere. They say they’re going to investigate it. The next day I get an email from T Mobile stating that they don’t have it and I owe $220 non-return fee  at this point, I am very angry. I get someone from T-Mobile on the phone. We spent close to an hour on the phone again today ! At the end of the conversation, I have gotten nowhere. They’re paid to be nice to you. But regardless of what proof I have, they are not changing the fact that they are saying I owe them $220. This is very frustrating. I want to switch back to my old company which was Verizon. Unless they correct this, I am switching back. I’m giving them a chance. But they need to let me know that this is resolved and that I do not owe them this $220 non-returned device fee.