Disappointing service from MTN

I'm so disappointed in MTN with the poor service received from them. I took out a contract with Mondo and within the cooling off period, I cancelled it as I was unable to connect. They advised me to take the router to the nearest MTN store, which I did. Shockingly, my contract was never cancelled on the system and MTN proceeded to debit my bank account. I got into contact with Mondo and got them to cancel the contract, which they then did. MTN still went and double debited me the next month as I had reversed the previous month's debit. Now they are sending me letters of demand and charging me early cancellation fees. I've never used their data, how can they still charge me for usage??? I'm trying to contact them to have the matter rectified but dialing 083135 has such limited options that there is no option to speak to a consultant. I don't even know what my allocated number was. How do I go about fixing this problem!!!!