Unfair Charges

I took a Cell C contract at menlyn Autopage for a data modem, the agreement was for me to pay R129 per month. two weeks later i got an email saying i owe R7500 for using out of bundle data, my first problem is that the modem could not display how much data i have used or how much is left, secondly i took it back to the provider they refused to exchange it or cancel the contract