Hi there! I see you’re trying to check in on your iPhone through the Frontier Airlines app, but you don't see the check-in option even though your flight departs in 16 hours. To get a better idea, could you tell me if you've had the app open for a while or tried restarting it? Also, are you signed into your account and have your flight details entered under “My Trips”?

Just to clarify, Frontier Airlines allows you to check in starting 24 hours before your flight departure. Since your flight is 16 hours away, the check-in window should definitely be open already1 2.

Here’s what I recommend you try step-by-step:

1. Restart the app: Close the Frontier Airlines app completely and reopen it. Sometimes, the check-in option may not load due to a temporary glitch.
2. Check for app updates: Go to the App Store on your iPhone and ensure the Frontier app is updated to the latest version.
3. Sign out and sign back in: In case the app isn’t syncing your trip correctly, sign out of your account and then sign back in.
4. Navigate to 'My Trips': Enter your last name and your confirmation number manually in “My Trips” to locate your booking. The check-in option should appear there once your flight is within 24 hours.
5. Restart your iPhone: A quick restart can fix minor software conflicts.
6. Try checking in via the mobile web browser: If the app still doesn’t display the check-in option, you can try accessing Frontier’s mobile check-in page at https://www.flyfrontier.com/travel/my-trips/check-in/ through Safari or another browser on your iPhone.

If after these steps the check-in option is still missing, could you let me know if you’re seeing any error messages or if the app behaves unusually? That would help me guide you further.

Are you comfortable trying these steps and letting me know where you get stuck?