Have you read this? https://community.home-assistant.io/t/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/