Breach of contract by giving a customer less data than what is stated on the contract

I took a sim-card contract July 2016 where I am supposed to receive 10GB to use anytime and 20GB night express. The Simcard is in a router and I am unable to access the SMS's as my phone uses a Nano sim and the router uses a normal sim. On the 14th of November I received data and a week later I had connection trouble and I called 808 where I was told that ALL my data has been depleted, including the 20BG night express. This seemed impossible to me because I never deplete my Night express, the call center agent told me that I received 14GB in total on the 14th of November and that according to their system I am to receive 10GB anytime data and 10GB night express. This is very infuriating because for over a year I have been paying for services/data which I was not receiving and I dont understand why I received 14GB, where 4GB was for night express. I could not see how much data I was receiving ever since I entered into this contract and MTN has been giving me less than what I was paying for this entire time. Worse part is that I was using my MTN prepaid to top up data meanwhile I have a contract which is robbing me. I was sent to MTN's retensions department and I expressed my disappointment and informed them that I was to cancel my contract ASAP and I dont expect them to charge me an early termination fee because they did not honor their part of the contract which requires them to give me 20GB which they have been giving me 10GB. There is no way they can go back and give me the 10GB they've been robbing me. I was advised to go to the store where I took my contract as they are the ones who did not load my contract correctly, when I got to the store, I was told they dont know whats happening and the person who loaded the contract does not work there anymore. They did not cancel the contract still. The reference I was given is: 1- ********** 3064 and they still did not resolve my problem and there was no way forward.