Mtn contract

Hello I took a contract with MTN in May the advertised deal was 40GB ( 20GB for any time and 20GB for night) its been 3 months now every time they load my data they just give me 30GB ( 10GB day and 20GB for night) I have been calling their call center, going to the store where I took this deal but nothing is fixed. Every time it’s a different story. When I say I’m canceling my contract as what they giving me is not what I signed up for they tell me I can’t. I’m still liable to pay for 24 months. If this is how MTN treat their customers then I might as well move to another network