Ok, so a few things: you need to make sure that some external port is forwarding to 5060 UDP (assuming pjsip) on your PBX.
Then you need to update the firewall on your PBX to allow external connections from your remote device(s) AND you need to place them in a group with access to your system. I recommend OTHER:
You’ll need to grant the OTHER group SIP access:
Under Services:
Make sure Other is selected:
