Loading of incorrect contract deal

Last year in April I was offered a deal by mtn which I took. I was supposed to get 40GB on the simcard for around R250 p/m so I requested 2 simcards. In May I noticed that the 2 simcards had different things loaded on them one had minutes, sms's and 5GB(I think) and the other was only loaded with 20GB instead of 40GB. I lodged a complaint, I was told there will be an investigation so I called back after 14 working days and findings of the investigation was in my favor, they loaded the wrong package for me and I've been requesting for a contract early cancellation fee with no luck and I'm being billed this whole time. I've been trying to cancel the contract since June last year. I'm very frustrated with all this.