Bad experience

I had a 36 months contract with MTN, which I never missed a payment for and it was set to expire on 25 March 2022. I called their retentions department on the 28th of March to cancel my contract and I received a reference number and an Sms confirmation that my cancelation was being processed and will be officially canceled on the 26th of April 2022. However, to my surprise my contract continues to be charged and I refuse to pay for something I commanded to be canceled and on time. I am now grey listed and also is affecting my credit score. I am so tired because I have been making a followup on my case since then but with no win. I need help