Bad financial service

I'm very disappointed yet again with Cell C, ever since in 2017 I took out fixed contract with them I have always been paying over and there was one particular month I paid in Advanced and they came back telling me my account is owing, every month been paying extra and still they will demand more monies , I asked them to check they say anything after 90 days they cannot help that was unfortunate...In early 2019 my contract ended and decided to renew it with data and I was told nothing was outstanding in my contract it is up to date then I took a go ahead of renewing my contract that I decided to cancel , now that I'm canceling they telling me that I have amount outstanding that was supposed to be paid in March 2019 which is Already September 2019 that is way after 90 days and why I was not told about ? I'm need them to go through each and every payment I made and actually reimbursed me all the monies were overcharged