Zigbee is a mesh network, and requires a routing device like every 25 feet to have a robust network.
If a device id ‘flakey’, it probably means you need a smartplug plug or other routing device between to help it network. Like 2 zigbee devices per room seems to work well in most homes.
That is likely not a coordinator because Zigbee can only have one per network. It would be the same as plugging any mains powered routing device in the area. See more in the Cookbook Zigbee section: