@Londonist 

Hi,

How much speed do you get when you are connected to the Wi-Fi network of the Zyxel router?
 

  @Londonist 

Ok. If one wall causes the speed to drop to almost nothing, then there might be something special about the walls of your building.
Did you previously have another Wi-Fi setup that worked fine in the same environment? (I mean before you got the Zyxel and the Deco's)

There is no reason to believe the Deco's Wi-Fi would fare better in penetrating walls than the Zyxel router. 
Are the neighbours you mentioned living in the same building as you or in a separate house next to yours?

To rule out a problem with the connection between the Zyxel and the Deco that is plugged into the Zyxel you might want to perform following test:
- Unplug the power adapters of both the Deco's that are connected wirelessly and only leave the wired Deco powered on. 
- Then connect to the Wi-Fi network of the Deco and run a speed test while standing right next to it. 
- Check whether you get about 200 Mbps.
 

  @Londonist 

In case you have the Zyxel router and the main Deco placed literally side by side, then you should probably better switch the Zyxel's Wi-Fi off.

Other than that, a Wi-Fi network that is just "there" but not actually being used by anyone shouldn't be a problem.
 

  @woozle thanks for your suggestions.