I used two month mtn data which is 150 monthly ,300 in total of two month which i payed,but I'm owing ridiculous amount

I app**** for data with mtn ,our agreement was to pay or debit 150 monthly,,end of the month 450 was debited from my account when I tried to complain, they dismissed me with" hold the line "until it goes off,they never solve anything

I asked them to cancel my contract within 3 month of opening an account in exchange of paying the 2 month I used,which I payed ,but the account is still open with the ridiculous amount that I don't even know where it's coming from.