Cancelling mtn contract is a nightmare

I took sim only mtn contract where we get 30 GB to use during the day and 40 GB for night, apparently the contract just switched i now get 10GB for day and another 16GB, nothing for night, i tried to query this but no one could give me an answer, now i am trying to cancel the contract the first consultant transferred me to cancellation department where i have been on hold for an hour, i just want this contract to end because i am not getting what i am paying for.