Misleading advertising and bad service

In February we decided to downgrade one of our cellphone contracts and take out a new router contract. We went to MTN Brits Mall and was helped by a lady called Natasha. We went through the brochure and decided on the 40gig Huawei router of R345 pm. On signing the contract we did notice it said 20gig 10 day and 10 night by she assured us thats just how its loaded as its a promotion. Since the first month we only received 20gig and could only use the 10gig day as we dont watch tv after 12 at night. We have complain numerous times at the store with Natasha and also Thomas and the only responds we get is that we are not the only customers that complain about it. The consultants in that store have no clue how to deal with customers queries