Do I have to be connected to Wi-Fi or a cellular network to use the Mobile Passport App?

Yes.

All airports and cruise ports associated with the Mobile Passport App have accessible Wi-Fi. Ease your arrival by using Mobile Passport plus to set up your passport profile(s) days in advance and without being connected. Then, upon your arrival in the U.S., turn on your Wi-Fi or data, make sure the Date & Time settings for your device are set for your current, local time zone, and use the app to submit your itinerary to U.S. Customs and Border Protection (CBP). They will send you a digital receipt in the form of an encrypted QR code after they have reviewed your submission, which will expire after four (4) hours.