High invoice billing

I took a mifi router contract with MTN last year, for 80GB per month I was paying R290 this in April it was increased to R307 which I didn't mind. I have been paying that amount ever since. When ever my data got depleted I would purchase additional data for the sim card. Last month I received an invoice of R1060 of which I enquired about they said I used it for my data usage since my data was completed. My bill limit was not on zero and I was charged from the limit they have put. Firstly my data has been depleted before and my router would want me to sign in and say out of bundle. So why suddenly I am being charged for the agreement I never had with MTN. Now I have received another invoice of R1600 while I told them to put the limit to zero, and they said they can't because I was already billed this month, why now do I have to pay R1600 while I was taken from pillar to post about the invoice and no one is giving me a valid reason.