Invoiced 9 months later on a contract cancelled (and confirmed cancelled by Cell C)

I have cancelled my fibre contract on August 3rd 2020 (first working day in August) and paid upfront for the month. I received a confirmation of cancellation from Tebogo at *** confirming the contract will be cancelled at 31 August 2020 with a claw back fee of R 0.00. 9 months later I get a new invoice generated in May, a subsequent final demand for an outstanding amount on that same account from the new invoice generated in May. I have tried numerous time to have this resolved as I do NOT OWE Cell C anything. I cancelled a month to month contract according to your cancellation policy requirements, I vacated the premises at the end of August 2020, hence the cancellation of the fibre contract. I need to have this resolved today! I need the final demand removed and my credit record restored!! I am truly disgusted by the incompetence I have so far experienced.