Breach of contract by mweb

I received an email from mweb that my contract that I have with them that I paid R478/month have been changed by mtn as I was on on 15mbs uncapped .Now MTN BROUGHT IN THST AFTER 50GB USES THEY GOING TO THROTLE THE USAGE . THE ORIGINAL CONTRACT DID NOT INCLUDED THIS AS I SEE IT AS A BREACH OF CONTRACT FROM MWEB SIDE.I DID NOT AGREE ON SUCH A CONTRACT .AS MY SECURITY SYSTEM WAS RUNNING ON THIS NETWORK I CAN NOT EVEN USED IT NOW.THIS IS TOTALLY UNACCEPTABLE FROM MWEB TO ALLOWED THST MTN BREACH THEIR CONTRACT WITH A CLIENT