Well, you could look for manga on platforms that offer offline download options. For example, some subscription-based services allow it. Just make sure you have enough storage space on your device and a stable internet connection for the initial download.
You might find offline manga reading options on dedicated e-readers like Kindle or Kobo. Some manga publishers also release digital editions that can be downloaded for offline viewing. Also, check if your local library has digital manga collections available for borrowing offline.