Wifi range is poor, what will help?

[#1]

We live in a new build property of timber frame construction. Every wall is installed with this silver faced insulation. The property is fairly small with a good sized rear garden. I'm constantly having problems getting the wifi to reach garden shed which is approx 20 metres away from the router. I initially blamed the poor Sky router so this time I've purchased the Linksys wrt3200acm. But still, despite its four alien looking antenna's have the same issue. 

I guess Ideally moving the router to the upstairs bedroom may be the answer, but there is no adsl socket there plus all the other devices are routed to the living room where the router currently sits. Any suggestions, is there a way to amplify the signal to get more gain?

[#2 · Accepted solution]

There are two solutions. The first and the best is to install CAT 5e Ethernet cable between areas where you need internet. You can then fit local Wireless Access Points at these locations. If this is not practical then another option is to use Powerline units that include WiFi transmitters. However you need to make sure that both the transmitter and receiver are on the same circuit. To check this you could plug an appliance into the socket where the transmitter would be located then switch off the MCB in the consumer unit. If you then plug an appliance in the socket where the receiver would go and this is also dead then you know you are on the same circuit. If the transmitter and receiver are on different circuits you will have problems. 

Here you will find some options: https://www.techradar.com/news/the-best-powerline-adaptors