Vodocom ripping me off

I entered into a 24 month data contract including a router at Vodacom Woodmead on 28 March 2019. It was a special deal offered at R120.17 PM.for 24 months. I have a signed contract! From April 2019 they have been billing me at R158.00 PM. 32% more than my signed contract value. I have have lodged various complaints to Vodacom. They have advised me that they can increase my contract monthly charge whenever they want based on their standard terms and condition s??????? I have an agreed 24 month contract at R120pm and from month 1 they are charging me at R158pm. ..a 32 percent increase on my agreed contract price. The call centre staff are rude and keep saying that I must refer toVodacoms standard terms and conditions.. What do I do???