Mtn **** service

I wen into mtn in January as i do every month and ask how much do i need to pay the consultant told me X amount which i paid after in the month after the bill run but a few days before the 29th of January i cant remember the exact i upgraded the contract which was on a month to month to a iphone 12 pro max. When i go in to mtn as i do every month i ask how much do i pay now they first didnt know then they tell me i have an over due amout so i must pay R2075 what the hell how due i have an over due amount when i come in personally every month and pay no one ever told me about an over due amount even when i upgraded no one said nothing. How did i have an over due amount and not be told plus i pay every month. How can i upgrade with an overdue amount that makes no sense.