What does your "Find My iPhone" say?

Anyway the flight number is useless to know. planes are rotated left and right, and after some time, absolutely 0 chance the phone is still in the plane.

If you were lucky, someone turned it in in FRA:
https://www.frankfurt-airport.com/en...n-of-loss.html
or MUC:
https://www.munich-airport.com/lost-...-center-622905

If they tell you they didn't find it, then your only hope is reporting it as lost or stolen to the police, who would be able to have the IMEI marked as stolen and potentially recover your phone - however it would have been lond since wiped. Plus, chances are, if it was found by someone who wanted to resell it, chances are they would do it outside Germany or even Europe anyway.