Contract fee changed unfairly

A couple months ago I wrote here but no one got back to me. My story about Vodacom is very unfair towards me, last year Vodacom phoned me inquiring if I would like to take out a contract for a router at a cost of R148 a month. This was a promotion they where running due to the Data prices where going to increase , I was told I would receive my router plus the sim and I would get 5 gigs of Data a month, I was also told that when the Data price increase I will still be paying my R148 NOTHING MORE AS THIS WAS A 24 MONTH CONTRCT. All was well I got my router and my Data every month for R148, but then it was around March this year I got the sms like always to say my bank account is going to be debited which was fine but instead of R148 the price was pushed up to R158, when I phoned Vodacom to ask what was going on I was told that they did send a email to me which I did not see as it was sent to the routers number and I only connect to the router for whatsup and FB with my phone, they said they said in the email the price was going up and they have the right to adjust contracts prices. so now im paying R158 instead of the agreed price of R148. A contract is a legal binding document and both parties need to stick to the agreement until the end unless agreed for changes by both which was not in my case, so it seems in short this law does not apply to Vodacom contracts, even if you sign and agree they can change and charge you more for the same service as it suites them. Are they above the Law. one thing I can say is when my contract ends the is the end for me dealing forever with Vodacom