Incorrect billing and charging

I have had an existing contract with mtn and only to find out out the 12/11/2025 that my contract was moved on to a month to month basis and I was slammed with at R1700 bill from previously paying R450.00 I have called in numerous times an queried this and was advised that the back office will come back to me and it being one month later I have no response from them and now can not upgrade my contract. I was failed to even be advised that if I don’t renew or cancel I will be billed on a month to month basis as I’ve always renewed on Black Friday and my contract was signed in 26/11/2022 for 36 months which legally only terminates on the 25/11/2025

I have numerous BO numbers and case references and call recordings but no response and each time I call they say someone will call me or will email be but 30 days have gone and I’ve called numerous times with the same answer

Could someone please advise a way forward because I am frustrated and would rather have my contract cancelled and I have other family members and friends that have also been slammed with high bills with failure to advise if not upgraded by a certain day you will be billed month to month hence one knowing that information will obviously renew a contract