Unable to upgrade due to triple amount billing

Hi I have a 2 year contract with MTN which is coming to an end on 22 November 2020, i have been paying R259 since i upgraded in 2018 and currently up for my upgrade. MTN has been calling me since last week to notify me that im up for upgrade but i told them that i will wait for Black Friday deals to upgrde.

Yesterday MTN sent me a statement and invoice for R724.54 which i was confused what is was for, i called them 3 times and spoke to 3 different people who each gave told me 3 different story, they basically told me that just because i didnt cancell or upgrade before they invoiced me i must pay the R724,00 amount (which i dont even know how it works)so i really dont understand why would they make me pay even if my contract has come to an end, i was never told i need to cancell before certain period, also they never sent me anything to tell me that if i dont upgrade before my contract ends i will be billed for whatever hell they billed me

Thank you I want to cancell that R724 they billed me so that i can be able to upgrade, because i wont pay this R724 amount the sad thing is that it will give me a bad name on my credit