How to Start the Isle of Siptah DLC
Edit the Engine.ini file through the web interface and add the following under the line:
[/Script/EngineSettings.GameMapsSettings]
ServerDefaultMap=/Game/DLC_EXT/DLC_Siptah/Maps/DLC_Isle_of_Siptah
To start the server WITHOUT the DLC again, change the line back to:
ServerDefaultMap=/Game/Maps/ConanSandbox/ConanSandbox