Unhappy Customer

I got a router in July 2018 (2-year contract). I paid every month and there was a month were they debited more than R800. They told me they will rectify it but they didn't. The following month they debited more than R600 and I disputed the debit order. After that I get a message saying I owe R2058.27 and my contract ended in July this year. I don't know what's the problem.