Contract canceled, but still charged

I took out a month to month contract with rain in February 2021. I have tried numerous times since Aug 2022 to cancel my month to month contract, but with no success. Every month the bill got larger even though I tried to cancel and did not use the service. Eventually end of 2022 I came to hellopeter to lodge my complaint. Rain actually answered me on the hellopeter site and indicated that they will sort out the problem. Today, after another year, I received an sms indicating that I was handed over to NuDebt. As far as I am concerned, the contract was canceled long ago. Why am I still liable for payments for a service that was canceled and not even used?