From Steam, right-click Subnautica and choose Properties.
Go to the Betas tab.
Next to Beta Participation, select legacy from the dropdown.
Wait for the installation to finish.
Download Subnautica Nitrox here.
Unzip the downloaded file to a new folder.
Run the NitroxLauncher as an administrator.
From the Nitrox Launcher, go to the Options tab.
Confirm the installation directory.
Go to the Play game tab.
Click Play Multiplayer.
Note If there is a runtime error, switch Subnautica to the legacy version. Learn how to here.
Press the Multiplayer button.
Choose Add Server.
Enter the desired name for the server.
Next to Host:, paste the IP from Step 3 without the Port.
On the Port: field, input the Port from Step 3.
Click Add.
Select Connect to the server name from Step 13.
Enter the desired player name.
Press the Join button.