Other than adding more (known good) Zigbee Router devices and using a long USB extension cable to the Zigbee Coordinator radio adapter you should also connect it to a USB 2.0 port, and not a USB 3.0 port because they cause serious interference for Zigbee signals/communications, (or even near any other USB 3.x devices like external USB disks or appliances/adapters/cables/ports), and since most Synology NAS usually only have USB 3.x ports you should really connect the Zigbee Coordinator radio adapter via a powered USB 2.0 hub since that will convert the port to USB 2.0 for it. Again see → Zigbee networks: how to guide for avoiding interference and optimize for getting better range + coverage