MTN waste of time

Almost 2 years ago I took out a data only contract for 20 gigs daytime and 20 gigs night time. For the past 3 months I have only received 15 gigs daytime. Where is the other 5 gigs? I am fed up trying to get through to MTN call centre. The probably would not be able to help anyway. I also dont expect anyone to reply on this forum as it seems by other complaints I have read that MTN only want payments and not complaints. We have 3 contracts that are due to expire in the next couple of months and these will be cancelled as soon as possible. Hopefully MTN wake up and sort out the massive problems they have or the soon will have no customers.