Getting internet reception

I have a computer upstairs in one rm. of my house with a combination modem & WiFi Router (DOCSIS 3 cable modem, 4 ;port Ethernet router and dualband WiFi). I recently put a wireless computer downstairs in the other end of my house (recently added room--went through brick wall--an addition. Since I could not get online I bought the above router which did not solve my problem. I then bought a WiFi range extender. I was able to get on line using the extender but after 15 min. or so, it would cut out. What are my options to solve the problem?