MTN Data disappear with excessive data billing

I have had a dispute with MTN for the last 6 months with regards to data bundles that disappear and then come end of the month receive a 300% increase amount owed to normal billing. MTN can not explain or show where purchased data bundles goes to. I have had in one instance purchased a 2gig data bundle only for it to expire the following day, i have monitored my data bundles daly to make sure i don't run out of data but yet i get charged every month out of bundle data rates even though i have purchased data bundles. I have been in contact with MTN since April 2015 trying to resolve the issue but to no avail, MTN does feel they are correct and keep on deducting excessive amounts of my account and i am sorry one will have to use a lot of data to receive a R14K data bill. I have been a loyal MTN customer since 2000 but enough is enough. I have heard from various other MTN users experiencing similar problems.