FYI -

I was digging in the program files for RealFlight and I found the file named "HostBroadcastPersistent.ini"
In it is your broadcast info.
The IP address that is in the file is actually my WAN IP address (public).

So even though Realflight shows my LAN IP address, my WAN address is actually published to the multiplayer server and people can join my session.

I tried it and it worked.

If your LAN IP address is listed in this file, manually change it to your public IP address using notepad and you should be OK.