Incorrect contract that mtn refuses to rectify

I have a problem with a contract I took online with mtn sale( monde). I took a 20 gig contract with them which was suppose to be 20 gigs anytime and 20 gig night. At that time they also had a once off promotional data that they gave me which was also 20 gigs free but broken up into 5gig for YouTube, 5gig for anytime and 10 gig for video streaming. The first month I received everything as promised... But the next month, I now started receiving the promotional data of the 5 gig 5 gig and 10 gig instead of my normal 20gig anytime. And mtn is blaming the sale department and the sales department is blaming mtn and I'm going back And forth. Mondo even sent me the contract and an email stating what they signed me up for but mtn refuses to fix the problem. Issue with receiving the promotional data every month is, if u don't watch videos, u waste 15 gigs every month like I am doing and you only get to use 5gigs of data for internet. Which is not what I signed up for.

I need help please. I cannot even change my package nor cancel it