The Journey web app is a web-based curriculum, so you need wifi or some form of internet access for your work to save. However, if you are logged in and already have a course you have been working on, you can work in The Journey web app without wifi. Be sure not to log out of the web app or refresh, and the next time your device connects to the internet your answers should sync and save.
If you know that you will be away from the internet, you can also download the appropriate units before you leave.
You can download a PDF of the unit by clicking the "Generate PDF" icon on the left side of each unit (see the example below. The download icon is the one above the "T" in a circle.)