You can add data while your device does not have an internet connection (offline), and the app will store your data for you until the next time you are connected to the internet. The app will automatically sync the data uploaded while offline to the algorithm and update your fertility status.
When you are in Offline mode, the app will show you an orange color, and will show 'Use protection' or 'Offline' depending on your goal (Prevent/Plan a pregnancy).