1. Make port mapping for communication ports of PBX on router. The default ports are:
UDP (SIP): 5060
UDP (RTP): 10000-12000
2. Go to Settings →PBX→General→SIP→NAT. If you use static IP for S-Series VoIP PBX, please select “External IP Address” on “NAT settings” and input the public IP address of S-Series VoIP PBX. If you use dynamic IP for PBX, please input the domain in “External Host” field on the NAT settings page.
3. Got to Settings→ PBX→ Extensions. Choose the extension that needs to be registered remotely, click. Select “Advanced” page, enable “NAT” and “Register Remotely”.
4. Register to this extension configured.
For more configuration details please refer to this link: https://support.yeastar.com/hc/en-us/articles/229524467-Configuring-Remote-Extensions