IKEA also sells an inexpensive purpose specific Zigbee range extender (i.e. Zigbee router) that is called “IKEA Trådfri Signal Repeater E1746”. It double as a USB-charger and supposedly has much better range extender functionality than the standard power-plug from IKEA.
Tip on a popular inexpensive DIY option for making a very good Zigbee router is to buy cheap CC2530 module or CC2531 Zigbee USB sniffers (be sure to get a model with an external antenna) and then flash them with the custom Z-Stack router firmware from Koenkk of Zigbee2mqtt fame:
Other general tips for improving the range and communication within a Zigbee mesh network
Add more routers between the problem devices and the next nearest router. In a Zigbee mesh network, each router will extend the range of the mesh network. As a rule of thumb, almost all permanently powered devices will serve as a router (however many Zigbee light bulbs are commonly excepted from that rule).
Adding a USB extension cable will move your USB stick away from the computer you’ve connected it to. . A USB extension cable of 1-meter and placing it away from any other radios/antennas can often reduce interference, and thus improve your range.
Add an external antenna to your coordinator, and position the antenna away from the computer. Next level of this is to add an antenna extension cable but be warned here that you should not use too long antenna extension cables as each meter added to decrease the signal strength.
Try different orientation of your Zigbee antenna (or your whole Zigbee adapter if it has an internal antenna). This is because the connection between the Zigbee radio of your “coordinator” and your other devices depends on the way the antenna oriented in space.
Ensure that any WiFi router/access point, WiFi USB stick, Z-Wave stick, or RF433 stick are as far away as you can get them (preferably at least one meter if you can). This will also reduce interference.
Check the WiFi channels in use, to see if your router/access point is now talking over your Zigbee mesh. If so change the channel of the WiFi access point and lock it. If you have to change the channel of your Zigbee mesh then you’ll have to re-pair your devices.
Buy more powerful Zigbee radio hardware with better radio range, preferably with an external antenna.