Hi and welcome,
make sure that you have created your lobby account within the game first of all - after accessing the Multiplayer from the main menu click REGISTER on the top to do that (as shown
HERE). Only once you successfully registered for the lobby once can you then log in with those credentials.
And just in case you still can not join the lobby make sure that you're trying to access the correct lobby since the default SimBin lobby is offline.
In your GTR2 install directory, if you open the LOCAL_IP_LOBBY.txt file it needs to say lobby.koelschbierbude.de, if that's not the case you need to download and install the
Lobby Patch, which however should be automatically installed when using our Autoupdater.