How to set up a formal complaint - I've been overcharged for well over a year and have spoken to multiple customer support representatives

Back in February 2025, I returned my Xfinity Wifi router having bought one myself online which is compatible with my home cables. I followed the instructions given to me, and returned the Xfinity router in person, to my local store. They made note that the unit had been returned and told me I would no longer be charged the additional router rental fee of c$25 per month. The next month, I was billed for another $50.88 (same fee as before), so I phoned Xfinity, the customer rep acknowledged the issue, told me they'd waive the next automatic fee, and I wouldn't be charged any more. I was then charged $50.88 again, every month through until now. 
Every time I speak to the customer support desk, they now say "Yes we acknowledge the issue and you're right, we'll credit your account" or "we'll stop billing you for X number of months". 

Given it is now April 2026 and I'm still being charged for a Wifi router I've not had for 14 months is beyond a joke. What do I need to do to get refunded for all of these additional charges I shouldn't have been charged for?