Cell c: incorrect billing and unfair charges

We purchased a cell c modem with data as we were moving in and wanted to stream. Ever since I took my contract with cell c I have been having NON STOP Problems! Firstly got the modem and sim, wanted to connect up but no Internet or only 2 bars. Phoned customer service / went to cell c shop to only have being told our firmware on the modem is out dated.. How can this be?? It's a brand new contract???? Changed modems, luckily my dad had a spare. Went well for a month or 2... We wanted to watch movies and use the Internet with the error message site cannot be reached from before the public holiday in September. Our modem was off, tried every now and then with the error message appearing again and again. Left it till the end of the month as our data will them reset. Received a surprise invoice. I owe them an additional R500. How can this be if we did not even use Internet and was not able to use anything at all!!! Tey and explain this to them. I had absolutely no luck. They are arguing that we used it when in fact we could not use anything at all. Once again we are trying to watch movies tonight and there is no Internet at all!! Same message as last time. Maybe id get a R1000 surprise account this month! I am so disappointed with cell c and have made it clear that none if my friends or family EVER use them!! Fine if I used the Internet I'd pay, no problem at all, but we were unable to connect to the Internet so this is impossible! Do not subscribe with them, the connection is SUPER bad and you get charged for something you cannot and did not use!