Breach of contract, ********* and dishonest

We moved into a complex, and like anyone else, wanted to get fibre. We went through Cell C. They sent us a contract and we chose the option we wanted (20mbp/s line through openserve). Our fibre was installed, but we now have a 10mbp/s line. We contacted Cell C, and went through to the JHB office where a lady informed us that we had a 10mbp/s line with Vumatel and not a 20mbp/s with openserve. The guy who organized the installation never informed us about the changes they made, we were unaware that they SCRATCHED OUT the option we chose and THEY filled in the option which is not the one we wanted and more expensive as well. They keep on saying that's the option we chose, but we have all the evidence, original contract, emails, Whatsapps, recorded phone calls in regards with what we chose. They are technically *****ing our money and they are in breach of contract. They recently informed us, after the installation, that our contract has been changed, that our Complex/apartment can only use Vumatel, but no one phoned us, or made us aware that this was the case and no one made us aware that the contract was changed by the person who organized installation. We are furious now.