Try to factory reset the device if possible (and that was not what you did in step-5), but it is of course possible that the device is simply broken, however I higly recommend that read and try following and tips here before troubleshooting any deeper as very common that EMF interference and/or too few Zigbee Router devices can cause weird and intermittent/random issues → Zigbee networks: how to avoid interference and get the best range and coverage
Note that included in those tips is also the suggestion to upgrade the Zigbee Coordinator firmware, though that is normally the last thing to do if all other devices are working fine.