Hi Maraia,
I just took a look and found that, if you tap your picture, you will be able to tap your online status (which appears in a slide-out menu) and change to Appear Offline. If you sign out (which you can do from the same menu that allows you to change your status), and then sign back in, your status should remain as Appear Offline. It worked for me, anyway… Although, I should mention that when I tap the the menu again, Available is the item that is checked even though my status states Appear offline.