For what? So you can shut down the homey itself?
So it doesn’t have to be 20 cm. Various devices of mine are up to 5 meters away from the Homey, even partly in other rooms. And these devices are directly connected to the Homey, without routing via other devices. But the range and quality of the signal depends on many factors.
The problem with the drywall could be that there are metal braces. That’s right, isn’t it? And metal braces disturb the Z-Wave signal more than stone walls (depends on the thickness, of course).
So you would have to try to reduce the distance or install more devices (possibly a repeater) in between, as Roco already wrote.
Actually I’m not a friend of repeaters (I don’t own one either), because they make the socket unusable. In this community I’ve read a lot of positive things about the Aeotec repeater. It’s not as expensive as another WallPlug.
But currently only the Range Extender 6 seems to be supported, the RE7 seems not to be supported yet.