Making promises they do not keep

Renewed my contract with mtn in August 2017. Took a special which included 3 months free 10Gb data. Then on the 31st August I get this sms from mtndata to say that because I did not have a full month of data on the the first cycle, they had extended this promo on my line and given me a fourth month of 10Gb complimentary data. So when this data did not load this cycle and I queried it, I was told the data was only valid for three months. So they **** to me about the promo - but I cannot say I expected them to honour their word because in my experience, mtn is not about the customer but about their profits.