I had a problem in that my phone said I had a signal but what I failed to know at the time is that I needed to sign in to the ATL wifi separately. I was already on the customs line when I realized my error so it wou;n'y have made a difference at that point. If I had realized that as I was walking from the plane to customs I would have saved about 30 minutes because the mobile passport line was about 6 people total.