Terminated Internet services

Took out a cellphone and modem contract years ago. So you would think that Mtn would have your contact details but no.... A couple of years ago, I bought out my cellphone contract but asked the operator to please continue the debit on my account for the modem. Last year, I came into financial issues and went under debt review. My provider switched all my debit orders to a different account. If Mtn was still debiting money from my account money, this would have changed and would be fine. But no, they never continued the debit. Instead they started deducting from the cash that I had paid for my phone to be switched to prepaid. 2 months ago they decided to terminate my Internet connection. No phone call, no email, NOTHING!!! Now I have over R1000 outstanding. As a student, how am I going to pay this? This is so annoying. I'm really regretting being a Mtn customer. I would like to speak to someone who can tell me why my instruction was not followed initially and why I am still being charged for a terminated Internet connection.