MTN Over-billing

My black Friday contract came to an end on the 25 November 2020, I was paying R 299 per month for my Black Friday data deal, due to my Father in law passing away and myself moving house I was unable to do upgrade this year On the 12 November 2020 I was sent my latest bill, MTN has billed me R 1 630. 67 and not my original contract price of R 299 which is unaffordable, due to this I will be forced to cancel my contract with MTN as the consultant at MTN has told me I have to pay the bill of R 1.6k Dear MTN, you will be loosing me as a customer and many other people, terrible service from MTN.