A saved title is not a video file
My List keeps a reference to a movie or series. Its backup contains those references and manual checkpoints. It does not contain the film, and opening that backup offline cannot play a movie.
Check the viewing service
If the chosen provider offers downloads, use its supported app and confirm the title is eligible. Download availability may vary by plan, device and edition. Read the storage requirement and expiry information shown there.
Before leaving your connection, confirm the download has completed. Check the preferred subtitle and audio tracks, then test a brief playback in that app. Seeing a poster in a queue does not establish that the whole video is ready.
Plan for the journey
A few appropriate episodes may be a better choice than several very long films. Leave room on the device for other needs and avoid changing account settings during the trip. A provider may require a later connection to renew an expired download.
Use movie-length filters to decide what fits the journey. Keep the backup for your choices separate from the viewing service's download controls. Neither a home-screen shortcut nor a list export replaces the latter.