Cell C Fibre problematic

I had fibre installed at my home by the fibre company on 10 Oct and then on 17 Oct the rep from Cell C came to install the router from Cell C. The problem is that I do not get signal in certain parts of my house. I have a 100 square meter house and my bedroom is 5 meters away from the actual router. When I called the fibre company, they advised that the signal is strong on their side and that I should contact Cell C. When I contacted Cell C they advised that I need to extend the range on my router. I looked on the back of the router and there is no IP address so I cannot access the router. The person who installed the router did not give me that info. I am using a tg789vac router. The person I spoke to did not believe me that there was no IP address on the router and was unable to assist me. Please could someone from Cell C call me back and help me with this problem.