Billing dispute

I switched from a 300mb down load plan with a 1.2tb data cap in late July this year (2026) to a 300mb down plan with unlimited data and a 5 year rate guarantee. I was told that my old plan had been canceled and that the new plan had been transferred to my modem-neither of which was done until a few months later. In other words both my old plan and my new plan were both active with my modem on the old plan. I received a bill for $209.46 including a $100 charge for data overage after I was supposedly on the new unlimited plan. I have called customer service about 12 times and was told by various agents that I was not at fault and should never have been billed and it would be taken care of-obviously it never was as I keep receiving bills. I have tried to be patient hoping it would be resolved but to no avail.