To play offline I had to follow these steps:

1. Turn off wifi on steamdeck
2. Start game and wait until you see the login screen
3. turn on wifi on steamdeck
4. Login in the ubisoft connect launcher (make sure that it is NOT in offline mode) and start the game
5. Wait for the game to start then close the game regularly through the game itself

Afterwards the game should be launchable both online and offline. However when starting offline the launcher will ask you to switch to offline mode (select with X). After doing this the application should start after a slightly longer loading time/black screen.

Switching between online/offline works without any further configuration then (however offline always requires you to go through the launcher.

Setting the "start launcher in offline mode" does appear to reset everytime I start the game.