Being Charged for an account that was canceled

I canceled my FirstNet plan in August 2021, at which time I moved overseas.  However, I kept being charged monthly, and attempted to dispute this multiple times with customer service.  FirstNet customer service kept forwarding me to ATT customer service, who then tried to always forward me back to FirstNet.  After trying this multiple times, I disputed the charges with my bank, who began intercepting the charges months ago, which were still in auto pay.  Nevertheless, I was just informed via email that FirstNet has still been attempting to charge me this entire time, and is now threatening to send me to collections.  

I am at my wits end, as I have tried calling customer service, emailing an ATT representative, and disputing with my bank, but it seems that FirstNet Is still trying to charge me for a plan I canceled over a year ago.  Can anybody point me in a direction that might help?