can i cancel my data contract if i do not get service?

I took a new data contract (110G anytime and 110G Night) in the last week of November and was given pro-rata data. The new cycle is meant to begin on the 13th of each month. By 24 December we ran out of data and when we checked our balance, there is only 5kb remaining. This does not add up as the night data should still be there (at least 80% as we go to bed around 11pm) but there is nothing. Now when i contact MTN, i am told it takes 48 hours to attend to queries. What kind of customer service is that? If what i suspect is true, who pays for the extra data that i am now buying? This is very upsetting and am considering to terminate since there is no service.