MTN not delivering what was promised

Data share contact not as advertised. I subscribed for a data simcard to be able to share data. The mtn employees told me I will be able to share the 20GB data, but after approval of the contract, I am not able to share 20GB, but ut only 5GB. I asked a few times to make sure I can share 20GB, and every time mtn employees said yes!

How do I get this issue fixed. I have called the store and made numerous attempts to contact mtn via email, but with no answer to my queries???