The Marvel Unlimited subscription service is accessed by streaming the comic content from the internet.
With the Marvel Unlimited app, you can download as many comics as you want, up to your device's capacity. These issues will be downloaded to your mobile device so that you can read them without an internet connection.
To download a comic for offline reading, tap the comic cover to open the comic detail page, then tap the "Download" button. That comic will be downloaded to your device. You will be able to see all your comics for offline reading in the "My Library" section on your mobile app under the "Downloads" tab.