The app needs internet access to post lap times, ghost cars and setups to the web server, and to get lap times, setups, ghost cars and standings information from the web server.

To be able to do this, you need to allow the exe in the app access to the internet.

The Assetto Corsa game folder is usually located here:
C:\Program Files (x86)\Steam\steamapps\common\assettocorsa

The exe is located inside the game folder here:
apps\python\SetupsAndTimes\app\SetupsAndTimes.exe

Please allow access to the internet for this exe in your firewall program.

See here for instructions of how to do this on the default firewall for windows 10.