Using a server that does not rely on NAT/Private IP ranges, and has 2 or more public IPs bound to it using Windows Networking, such as from a /29 range.
Windows Firewall on or off makes no difference.
Start arma3server.exe dedicated server up first, then the HC after it is loaded.
HC command line is:
-client -nosound -profiles=HC -name=HC -connect=24.100.55.55 -port=2302
Where the IP is the one set via the -ip command for the main dedicated server.