Freepbx NAT / Firewall blocking Remote Extension - Config correctly

Freepbx general support, particularly security, external sip phone connections like csipsimple not working, obi202 not registering, vpn connection setup, etc It seems there is NAT / firewall config issue between remote extension and freepbx. Access available via TeamViewer only and credentials will not be shared for security and privacy purpose. No add-on module or program to be installed.