Cell C data contract charges

I had a contract with Cell C for 20GB day and night data for R270. My contract was supposed to end in September and they called to tell me that the 10GB part of my 20GB deal was promotional in 2018. During this time they had increased my contract payment to R300 and also was contacted by their consultant this year that I'd have to pay R320 for 10GB day and night data, which I refused. They clearly never took the hint because they've been billing me R320 + R70 for non-payment for the past three months. When I called to cancel the contract they said I can't cancel because my account has outstanding fees which I didn't sign up for because I never extended my contract. Now, I have to settle an account with data bills I was not using because I haven't used their sim card since September.