Incorrect billing

I signed a contract 2 years ago on Black Friday with MTN indicating that I will pay R419 per month. The contract is coming to an end at the end of November 2020 and I received an invoice for November 2020 to the value of R2,187.67. MTN cannot show or explain this to me apart from saying it is pro rata? This makes no sense. I wanted to upgrade this month again, but I have serious doubts!