It used to be the case that you can check in online from 48hrs before your first flight, except itineraries to/from USA. I’ve noticed since last year they have changed it to from 48hrs prior to the second flight ex-Doha. They have now included this new rule in their FAQ page.

I find this pretty annoying, since the app or the check-in portal itself do not communicate this rule, and most often the departure time for ex-Doha flights are not convenient when converted to my timezone. That’s a hurdle for people who want to check-in early to secure their seats.