See the following recent discussion:
The Ikea Tradfri plug-in pocket sockets are $10 each when bought directly from IKEA and happened to be particularly good zigbee repeaters when used with SmartThings. I would add a few of those. Then unplug the hub and leave it off power for at least 15 minutes as @sidjohn1 suggested, and let your network rebuild itself when you power iton again. 