Hello Buggy
Maybe it is because you are using the file 'GTR2.exe' from your original install of Steam.
You need to have the file 'GTR2.exe' from DVD version.
Here is the link to download the 'No CD Patch GTR2.exe'
https://www.koelschbierbude.de/component/
option,com_remository/Itemid,0/func,fileinfo/
id,646/lang,de/
Rename your original 'GTR2.exe' to 'GTR2.exeSTEAM' or simply make copy of your original 'GTR2.exe' then put the 'GTR2.exe' from the 'NoCdGTR2' in your GTR2 Folder.
When you start the game do it using the GTR2.exe and not the GTR2 shortcut from steam.
Normally Then you should have access to your setup menu in Multiplayer.