You need Origin to play Sims 4. You can play it offline by logging into Origin.
When my internet goes out and I want to play Sims 4. I have to log in with my username and password and then I am able to play in offline mode. As it automatically gets put into offline mode.
Go to Game Library > Sims 4 and click Play.
With Origin showing a white box at signup that may mean their is something wrong with Origin itself and may need to be reinstalled.
If you have a laptop you can take it to a friend's house, local coffee shop or library to see if you can re-download Origin from there to see if that helps.
