Probably coincident in combination with interference being the real culprit and reason why can not pair, read and follow → Guide for Zigbee interference avoidance and network range/coverage optimization
Slightly off-topic; Tip! Can convert CC253x sticks into Zigbee repeaters by flashing it with Z-Stack Zigbee Router firmware (then using in a USB-charger for mains-power) so can pair it after that. See ex: