Why do you want 2 or maybe 3 different Zigbee networks (don’t know if you use Homey as Zigbee GateWay)? This can cause further problems.
I have also stone walls and it works. Maybe you need to place 1 or 2 repeating devices near the inside of the wall and 1 or 2 at the outside.
Another possible solution may be a strong repeater like this one (sorry, only in German).
