Very bad service at MTN

MTN south africa is robbing us.i recently bought a wifi router from them and our agreement was they will deduct R179.00 per month for 20GB after few months they start to deduct R459 when I ask them they said is the charges that I have been connecting to the router whiles the data was depleted. I ask them how can I connect to router without data because is the same as you can't make a call without airtime. I wanted to cancel it they refused to cancel but when I took it they said you can cancel at any time.