Atticus has some offline functionality, meaning you can write while offline and it will sync to the server once you are back online. You do need to be online to import and export files, however, and you'll have to make sure you're logged into Atticus and have the program open in your browser before you lose your internet connection.
To use the app offline, you have two options:
You can simply turn off your wifi/internet connection
You can disconnect the app from the network, which is a bit more technical, but this little video walks you through the process: https://d.pr/v/YNWG3Q. If you prefer text instructions, here you are:
Right click anywhere on your Home dashboard
Click Inspect
In the code box that pops up, click the two little right facing arrows
Choose Network
Click the down arrow beside No Throttling and choose Offline
This turns your connection off for Atticus specifically
To get your connection back on, click the arrow beside Offline and turn it back to No Throttling.
While you're offline, your work will automatically save to your local server, but when you're done for your session I strongly recommend you hit the Save button in the top right, next to the ? button, just to be sure everything force saves. This will also generate a backup file which we can use to restore the account, should anything terrible or wonky happen. When you go back online, it will sync up to the online server again.