Bill issue with my contract account

Good evening

I have taken a contract with Mtn at Northgate mall and My monthly billing wa supposed to be 199 every month but im finding myself paying more on what was agreed upon my contract, month of November was bill 413.97, i contact customer care call center on 26 November 2020 lay complain regard billing issue and they promise to resolve the matter within 3 working day , I havent received anyfeedback till now and got sms that my ref *** has been closed, I am very disappointed and angry and wanted to cancel my contract with immediate effect