Mtn poor service

I took out a contract in 2018 my contract was for a P30 lite 300rand a month for 24 months.. But they charged me for a p30pro that is 600rand a month because of a mistake the sales person made on mtn side in zevenwag mall I've been trying to stop the contract with no success if been speaking to the supervisor nothing is happening now I am debited every month then I have to reverse the money again the account is 15 months old but because I paid double the account was paid in full may month already please help me with this