Bad service

MTN is overcharging me for my pocket router and they giving me less monthly data when I call them I was told I did early upgrade on January mind you my contract is about to end this year may because is 24 month contract, now the telling me I did an early upgrade which I never did then I said to them they must cancel everything because I didn’t agree to it, now for me to do cancellation I must call service provider for cancellation and am going to be charged for that again Kindly assist me this matter