I had a WiFi router contract with Cell C and when the contract is about to end I was in arrears for 2 months towards end of contract and I called them wanted to pay for the arrears and end the contract and they refused

I had a WiFi router contract with Cell C and when the contract is about to end I was in arrears for 2 months towards end of contract and I called them wanted to pay for the arrears and end the contract and they refused. Now they sending me ridiculous amounts saying owe them. The agreement that I had with them was the premium of R292 and now they telling me that since I didn’t call to stop the contract the premium automatically goes up to R750 and now they say I owe them 6000 . Please assist