Not sorting out line speed!

I am sick an tired of companies that think they can ***** your money and think they can get away with it. I paid for a 40/20mb fibre line but they have capped the line at 10/5mb. I have been in contact with Bradwyn Pretorius from their support who has done nothing and decided to stop responding to the ticket I logged. I replaced my network cables and changed the router without any difference. He told me to plug the cable directly into a pc from the ONT and change some settings and check the speed again. It did not work because he provided me with outdated and incorrect steps to follow and did not reply when I told him it does not work or that he did not send the email he told me he has sent with the login details. I have sent more messages without response. They have done nothing from their side to sort it out. They are in breach of their contract by not providing the service I am paying for. They are commiting ***** because they are charging me for a service I am not recieving. They are not respondin in hope that I will just keep on paying without them fixing the problem!