I just bought 5x Deco M5 to cover WiFi in my house. 
 

I am a bit surprised to see that the signal between few devices are very low so I lose the internet speed substantially.

For instance, I have 3 of them on my second floor. The first one has good signal from 1st floor and internet speed is good. The next one is just behind the wall, 2 meter away and the signal strength is the lowest. I see in the app that it's connected to the correct Deco.

Why is the signal strength so crap just with a wall between? What can I do to improve strength?

Before I had the power line adapters to get internet around the house but it had its complications too.