Best WiFi Booster

Have you tried changing the wifi channel yet ? You can improve the signal by scanning all wifi and then selecting a channel different to everyone else near you. Its worth a go before you purchase a booster, it worked for me anyway.

http://www.howtogeek.com/197268/how-to-find-the-best-wi-fi-channel-for-your-router-on-any-operating-system/

If the problem is upstairs, it might be bricks and mortar getting in the way. You could try simply moving your hub, or the gadgets that piggyback onto your mains electricity cables.

^ powerline adapters

If your router has an external aerial with the standard connectors you can try plugging a better one in, if you've more than one connector and it's one area upstairs you could try a directional aerial. With a directional aerial you can be on 5 bars at a distance through walls. I've used a TP link square one, recommend it.

Other than that you can get adaptors that allow the signal to go through the mains then out again.
Even BT talk about them.
http://home.bt.com/tech-gadgets/internet/how-to-boost-your-home-wi-fi-11363806366107

Oh another idea depending on your layout and how easy it is in your house, if you do have a spare wifi antennae connector, is just to run say a 5m wifi cable upstairs and put a good antennae on the end of it, it'll be like router is upstairs, and not expensive.

http://www.amazon.co.uk/TP-LINK-PA411KIT-Powerline-Adapter-Starter/dp/B0084Y...

redcar Im not convinced your link is wireless it seems to use ethernet cables

Got a Develo and it does the trick.