desperate to extend a zigbee network. I've tried using Zigbee plugs, generic ones and sonoff ones, signal reach doesnt seem to get better at all 
So trying this one.
What device driver shall I use with this? hubitat doesn't seem to have a repeater driver for zigbee
A Zigbee router (repeater) devices does not need a driver to function correctly.
All that is needed is to pair the router to the hub.
If you want to monitor the repeater online/offline status, you can use one of the drivers listed here :
Search for "Zigbee repeaters (routers)" in the top post.
Download the Hubitat app
