I agree with Scunner regarding the electric circuits in your home. We are on 3 phase supply and unless the room in which your modem is connected and the room in which you want another connection are on the same phase you will probably encounter connection problems.
Earlier this year I installed a WiFi repeater at the top of our stairs so I could get a signal upstairs and it works so well I installed another out on our veranda last month so I can sit out in the sun and still get a great connection.
I also installed one for a friend so they now get a signal in all rooms of their Bungalow.
I got them from eBay for £16 each (buy now, not auction).