MTN just debited R2119 from my account and refuse to pay it back due to technicalities

I had a conract for a few years with MTN, which was a promotional data contract, I paid R322.13 for around 300gigs data. Last month my contract came to an end on 12 September. Like a fool I waited till that day and called them to say I'm not going forward with the contract because their network coverage has deteriorated in the area where I live. So the agent told be I have already been billed for the next month and that I should have stopped the contract 21 days prior to it ending, which I was not aware of. So yesterday I just get a message saying you have started incurring costs from your spend limit and Whoop they debit R2119 from my account(for 300gigs of data). I phoned in to tell them I did stop the contract but all they say is that the contract was not stopped 22 days prior to it ending so they cant do anything about it, they have already allocated the 300gigs of data and that it costs R2119 out of promotion. So MTN has forever lost a customer. I cannot afford to throw R2119 in the water for data that I cannot even use due to the poor coverage in my area. Never, never and never again MTN.