MTN 20GB + 20GB Night ****

I took the 20GB data plan with MTN but the data finishes in 6 days. I was told to wait until the 12th for a recharge which also lasted me about 5 days. upon investigation I realized the data isn't 20GB anytime + 20GB night surf as advertised.

How do I get out of this contract because this isn't working for me?