not sure nowdays but didn’t you have to have a twisted cable before linking pc’s on lans.
also have you tried just typing join server ip at the console of your other machine that used to work for me
regardless if server was seen online, esp useful for steam where private players servers could take 36 hours
to even show up on the private servers lists. tbh

@Royal-Davy Port forwarding is done in your Firewall application or possibly on your router; you can forward your ports by following these instructions;

@Reithur Thanks for the quick reply!