@David-Llado, there should have a scroll menu to select tank size, a first number from 1 to 30 to select litre and a second number from 0 to 9 for decilitre.
I have 3 Suunto plus app, The first one is always Marked as"false". When I scroll down, and then up… surprise the toggle was “true” just wasn’t showed as is when the first render.
Known, should be fixed with next firmware update as far as I remember