False advertising

MTN has an WiFi contract for R249 but first payment was R460 for activation was ok with it now second monthly payment is R680 I got a call from them to insure the router I canceled my contract on the spot as I can't afford more than R249 but now they threatening to blacklist. What do I do now. The debit order that suppose to go of is R450 more than the agreement its ridiculous so disappointed for there false advertising and the people at their call centre's such.