Incorrect billing for a contract that I paid fully

I took a 24 month data contract, 11 months into the contract the Sim stopped working, I contacted them and they said the technicians will call me and sort it out but they never did, I called every month since I was still paying and was told to go do Sim swap but still did not work, I tried to cancel the contract and they said they will charge me penalty fees, I decided to continue paying for the full contract term even though I could not connect to the internet, at the end if the contract, they put me on a month to month which I did not authorize, for something that is not working... I stopped the debit order and contacted them, everytime I call them they promise me that someone will call me to resolve the issue, I have never received a called instead they still continue to bill me and now they say I owe them R1900 but still no one is calling me about the Sim card connectivity issue or why I was put on a month to month without my consent