How to switch between SIM cards in Viking App?
If you use more than one SIM card connected to the same account you will be asked which one of these is default every time you log in to the app.
For the Android device, use the list in the top-left corner of the screen and pick "Balance". Then pick the card.
If you use the app on iOS just click the SIM card icon in top-left corner and you'll get a list of all active cards.