Still paying for a service 4 months after cancellation

October last year I took out a 4G contract with CellC which worked perfectly. But in April this year, we moved to an area where there is no 4G coverage. I was initially told by CellC agents that they were experiencing problems with the 4G sim cards, and that I should just be patient. I went to a CellC branch where I've been told that there is no 4G coverage where I lived, and I had to cancel the contract. So I filled in a cancellation form, but were told that the matter will first be investigated. I received a call from a CellC agent who told me that the investigation concluded that there is indeed no 4G coverage where we currently live and that I can either cancel the contract or change to another contract. This I had to do at a CellC branch. Again I went to a CellC branch, completed the cancellation form, again, and gave them back my 4G sim card and wifi unit. This happened on the 15th of May. Since then I've been billed every month for the 4G contract although it shows "Pending Cancellation" on CellC's system. On the 11th of September I went back to the CellC branch and inquired about the progress on the cancellation. The followup revealed that the cancellation couldn't be processed, and that I had to follow the cancellation procedure again. I was told that matter will be investigated again. It is now the 30th of September and haven't received any feedback of any kind from CellC related to this matter and so far I'm still getting billed for a service and unit I don't even have anymore. How difficult can this be?