@Gavin-Westfield suggest upgrade to the new ZBT-2 as Zigbee Coordinator as more importantly try to follow all these best practice tips → Zigbee network optimization: a how-to guide for avoiding radio frequency interference + adding Zigbee Router devices (repeaters/extenders) to get a stable Zigbee network mesh with best possible range and coverage by fully utilizing Zigbee mesh networking
Especially make sure to add more ”known good” Zigbee Router devices for better range and coverage.