Be careful OF MTN

I got the month to month wi-fi deal from MTN, for R399pm for the home internet and FREE router. It is 20mb/s. I did speed test on several occasions, it gives around 4mb/s any time of the month or day. Is this a ****? They also deduct R732pm for this deal and not R399pm like advertised or agreed on, is it classified as theft or *****?