4-5 days is more time without Internet than I'm used to, so other people may have better advice.

For using the Steam Deck offline, you can either use it in online mode (without Internet), or switch it to offline mode. For most people, it's best to just stay in online mode and never touch offline mode even if they'll be offline for hours.

However since you're going multiple days without Internet, I'm guessing you'll probably actually need to use offline mode. I would recommend launching any newly installed games in online mode once, then switching to offline mode (both in steam settings and wifi turned off) and making sure everything still works. It's hard to determine what DRM different games have, and what will work or won't work without Internet. Anything that you find doesn't work can be refunded if you have less than 2 hours of recorded gameplay, and you may have to rely on that mechanic to deal with games that have DRM that keeps you from playing on the ship.

Also worth noting, offline mode specifically means the deck is not connected to Steam's servers, but wifi and non-steam internet services can still work. Putting the deck in offline mode is supposed to keep you from having issues with the steam license check, but other DRM programs can do license checks as well and may fail if you don't have WiFi (regardless of if you're in steam offline mode or not).