To fix not being able to join yourself.
Insert both the IPV4 Address (CMD > Ipconfig) & WAN (which you can find here:
https://www.whatsmyip.org/ ) in the blank spaces located:
Local Disk ( ?: ) - ?:\Users\[User]\AppData\Local\Microsoft\Halo 2\halo2config1.ini & halo2config2.ini
If you wish to be absoulty sure it work, port forward (On your router) both:
2000-2010 TCP
2000-2010 UDP
Then apply the same for advanced firewall security by making new rule(s) for both inbound and outbound rules.
Ex.
Windows Defender Firewall with Advanced Security > Inbound Rules > New Rule... > Port > TCP > Specific local ports > 2000-2010 > Allow the connection > All checked boxes > Name: [H2PC Server 1] > Finish
Then repeat for UDP, then repeat steps for Outbound Rules. (Be sure to hit Allow the connection. "Block the connection" is set by default).