Unapproved upgrade

I had a very bad experience with Vodacom. I had a contract with them paying for router that was only 24 months and after that 2years I cancelled my contract with them because I was not longer needing their data the consultant I spoke to decided not to cancel my contract instead she upgraded without my approval now they claiming I have an outstanding balance of R600 and I’m not going to pay any cent because according to my knowledge I cancelled that contract with them. it was my first time having contracts with them and I regret