Cell C - Pathetic service & *****s money from customers

My WiFi router contract ended on the 25th August. In June already, cell c called me to renew this contract, which I didn't. I said to them that I will continue this contract month to month and they agreed to give me this contract at almost half the price than my initial contract. From September till now .... Which is the last two months.... Cell C has been charging me extra than the amount they offered me for the month to month contract. I have contacted them several times regarding this over charge, and they still continue to do this. Cell C can consider me cancelling this contract, because their pathetic service to me as a customer of over 10 years is now too much to handle.