Every single mains-powered Zigbee device is a wireless extender - remember this is a MESH network!
I’d suggest a read of " Network optimization":
Don’t think of Zigbee as point-to-point radio links like mobile phone macrocell towers as mesh networking works best with LOTS of SMALL devices that understand the network better than any human can. 
For Z-Wave, Zigbee, and, Thread, the answer is always more small mains-powered devices (and only very occasionally a firmware update or frequency change).
Mains-powered IKEA kit using Zigbee, is widely available globally such as the plug-in switches and bulbs. The battery kit (e.g. remotes) is less compatible and needs “quirks” to fix odd behaviour (firmware updates fix a lot).
If this helps, 
this post!