harassed after account closure

I was a T-Mobile Wireless Home Internet subscriber for about 2 years.  This was a no contract month-to-month account.  I called T-Mobile on April 12 2022 to close the account.  I was sent a mailing label to return the device, which I mailed later that week.

Fast forward to June, my bank account was still being drafted by TMobile.  I called customer service, and was told it was resolved.  I was also credited for charges after April.  I was satisfied with this result until ...…

August 26, I received a bill for not returning the device.  Again I called customer service, and was told it would be resolved.

September 30, I receive a letter from a collections agency for $172.  I spoke with Gwydolyn at T-Mobile who assured me that things would be taken care of.  She said I would hear results in 7 days.  I did not receive further assistance.

Dec 22.  I'm still receiving bills for "non-return".  I'm on the phone right now with T-Mobile.  Service rep said the device was received by T-Mobile on April 26.  I've been on hold for 20 minutes, waiting for them solve the problem.

To date I have about 3 hours of phone calls, and much anxiety and anger.  This needs to stop.  

Reference case numbers 64543684, 5196243229, account # 971245128.  I'm sick of it.