using the Steam -> Favorites
and use the query ip which seems to be +1 compared to the ip adress showing for the server.
another thing was i uploaded my "world" or savegame from my local game. the .db and the .fwl file.
located under: Users\yourusername\AppData\LocalLow\IronGate\Valheim\worlds
filebrowser -> valheim -> savedata -> worlds
and i gave it a minute after restarting the server
it seems you have to create a game fist and upload to the server as the the server didnt not work with the default world.