You can use Metronaut offline and without internet connection.
The first time you access a piece, it is downloaded and saved on your device. Next time you access the piece it will use the saved version on your device.
Starting version 6, pieces that are already available offline on your device will appear with a ⬇️ sign next to the piece. For example, in the screenshot below, Chopin's Nocturne is already available offline but Bach's Prelude isn't.