WRONG PRODUCTS OFFERD AND FAILING TO RECTIFY IT

i did a contract of 40G of data together with a Router , the agreed amount was R230 . ever since I started receiving the data I noticed that it doesn't last reason being they allocate 10G every month of which its not what I signed up for. but they still debit the premium that is not worth the data that they allocating for me. I have reported the problem to MTN TZANEEN LIFESTYLE BRANCH , they always say they have logged a ticket for me but they n ever resolve the issue , its been 3 months that I am paying too much money for a product that I did not sign for. when I say I am terminating the product they threaten to make pay penalties for something that is not my mistake at all, please help me .