unresolved matter

I went to mtn in October 2015 to open a contract for 2 phones, 1 was for R599 and 2nd 1 for R369 p/m.At that time I was promised 1GB of promotional data for 3 months 500mb data and 100 anytime talking minutes p/m, surprisingly I never got promised 1Gb for 3 months,I receive 300MB of data every month instead of 500mb I subscribed to and my bill R445 p/m instead of R369.I went to the store to lodge a complain since Dec 2015 till today my matter is still pending.I even spoke to the branch manager who promised that he'll investigate then send me an email, until today nothing has changed.Im frustrated now coz I'm paying way much more than the contract I signed for.Please advice me on which route to follow to try resolve my nightmare.