Mtn refusing to cancell my contract within cooling off period

Signed a contract on friday, 11 Oct 2019 for Data and Im told I used up 16Gigs from friday, 11 to Sunday, 12 Nov. This clearly is a *******. If Mtn data moves that quick it means 40 gig that I will be paying for will be finished in 1 week. I want to cancel my contract within the cooling of period which end today. I was at Mtn Montana yesterday but they did not hel. I need help because theses guys want the cooling off period to pass. Now I am told that they loaded 40gig, how did they load it if I requested cancellation. I WANT TO CANCEL THIS CONTRACT I AM STILL WITHIN THE COOLING OFF PERIOD