Make sure you have at least "Version 3.0.3" of the app!
To zoom into the instrument, tap on the "expand" icon at the top left.
Alternatively, you can rotate your device to landscape (iPhone only). The instrument will open automatically! Make sure that the "Portrait orientation lock" is not active in the control center of your iPhone.
You can now scroll the fretboard or keyboard horizontally.
To exit the fullscreen mode, tap on the expand icon top left or the ❌ icon. Or rotate your phone back to portrait mode.