You are going to need to add more Zigbee Router devices before troubleshooting any futher, even if that means that you first have to unjoin:remove a few Zigbee End Devices and join/add them back later. Start by reading and try to follow all the best practice tips in this guide first, if you do then your problems will likley go away or the problem symtoms will become clearer as remove low hangibg fruit issues: