Incorrect provision of data

I have a two year data sim contract with MTN which ended last year December. The terms of the contract was that they would load me with 40gig , 20 for day and 20 for night. However In 2019 the month of August, September,October and November there was an incorrect provision of data, Instead of loading me with 40 gigabytes they were loading 30 gigabytes. I have tried numerous times to bring this to their attention and up until now my query has no been resolved. Through frustration I have since stopped making payments and my account is now sitting on a legal status. to make mattes worse even if the contract term has ended thy still continue billing me for something I no longer need, I am being told that they are unable to cancel because of the outstanding amount, meaning that I am now accumulating more debts and I am unable to use the data since its not active. I want MTN to credit me the data for months mentioned above and I will gladly settle my account. I also want MTN to stop debiting my account and I want the contract cancelled. MTN is in bridge of contract and need to correct their error.