CANCEL MY CONTRACT< YOU DO NOT DELIVER ON YOUR PROMISES

I have a couple of issues all leading to one. I took out a Cell C 200 gig LTE A contract with a modem, from Heidelberg Cell C. I was promised that I would be able to roam on Vodacom should Cell C not have signal. Well That is exactly what happened. My Vodacom phone downloads at 22mb but Cell C only 1mb. When I tried to roam it did not work. Your technical team explained that my modem cannot roam on Vodacom. My salesperson at Heidelber Mall, who made the promise, said he will find another modem. After waiting and waiting an complaining, I was finally told that he can not get me another modem, and best will be to cancel the contract. I agreed as I will not pay another 2 years for something I cannot use. I have taken the modem back to Cell C Heidelberg mall, where they accepted it and the sim card. I have now asked multiple times by phone and by email to get proof that contract is cancelled, but I am met with empty promises everytime. Even stories that I told Cell C not to cancel? I need this contract cancelled ASAP!