The offline mode for the mobile app can only be activated one project at a time. Before activating the offline mode, make sure you are connected to the interned, so the app can download all the project data first.
In Project Details, click on the three dots on the top right corner to open up the Actions panel. Turn on the offline mode in the actions panel.
Sync data will keep track of changes made. When you are back in a place with internet connection again, toggle off the Offline mode then hit "Sync" to push your changes to the webapp.