Zycoo CooVox-U80 User Manual page 131

2. If you are planning to register remote extensions, you have to port forward 5060(UDP+TCP), 10001‐10500(UDP)
on your router/firewall. And NAT support should be enabled on Advanced‐> Options‐>SIP Settings page.
Both "External IP" and "External Host" should be your public IP address, or can be your DDNS domain.