Worst client service ever! Cancelling my contract after 20 years.

I have had a contract with MTN for almost 20 years. MTN incorrectly change my account to a SME Customer contract and after numerous months and complaint finally managed to change this yesterday. I now want to upgrade my contract but the system state I am only eligible for an upgrade in March 2019. It's 2020 already! I have already sent a complaint email and received a reply that they will assist within 24 hours. I have not heard anything from them. MTN's service is ridiculous to say the least. Not one person can assist you. You first need to phone another department and so you just end up phoning every possible call center and nothing gets sorted out.