No service provided but charged fully

I have been in contact with Cell C since 26 April 2021. I was offered an upgrade which never worked. I then contacted them and I got another sim, which still didn't work. After receiving about 3 new data sim cards, and still no luck with my wifi, I eventually went to a Cell C store and was assisted. I was told that the cards that hadn't worked had been cancelled and the new sim card would override those cards. However, since May 2021, I have been charged for one of the sim cards that didn't work. Cell C can see that I have not used any data on that sim card but I am still being charged for it. They keep on telling me its a contract - yes, a contract for a service which I never received. How can I be charged. I am giving Cell C a last chance to rectify my account now. Should this not happen, I am going to see a lawyer and will cancel every single contract I have with Cell C. I have been a customer for a very long time and have never had such shoddy service from Cell C. Rectify or I will take you to court.