Deduction of my Airtime instead of data bundles

I bought mtn airtime and data bundles at the same time but when I'm using the internet it keeps on deducting my Airtime instead of data bundles. I called customer care several times but they didn't help me or gave me any solution or refund for that. I even gave up. When mtn keeps on doing this I'm going to wait until my contract ends after that I am going to change to another network.