30GB of data on any LTE device but data expired

I upgraded to an iPhone 5s in January 2016, 30GB data was received just as they had said in all their magazines and online, unfortunately for me when i had 26.8GB left this data expired. MTN has used false advertising to lure me in to another contract. I asked the lady if this data would expire when I took the contract and she said no, so why is my data gone, further more, every time I fill data or airtime it just disappears. I use facebook, whatsapp, and update apps. I should not have to fill data due to MTN promising me the 30GB, what is the point in giving it if you are going to take it away!!! I should have taken a cheaper contract so I could afford more data, data goes on a daily basis, if I look at my iPhone data usage, the data disappearing is unaccounted for. Please MTN stop messing South Africans around with your lies and deception!! Just give me back my 26.8GB of data that I was promised.