Sonoff zigbee dongle going off line temporarily

Hi, welcome to the forum!

You could first check this if there is anything in there: 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

Which Zigbee integration are you using?