You can add data to the Natural Cycles app while your device does not have an internet connection (offline).
The app will store your data until the next time you are connected to the internet, at which point the offline data will automatically sync, and the algorithm will take this data into account to update your fertility status.
If you have added data while in the Offline mode, the app will show the text 'Use protection' (NC° Birth Control) or 'Offline' (NC° Plan Pregnancy) in an orange circle.