The app and seat reservations are completely separate. You show both to the ticket inspector.
You only need wifi/mobile data to activate the pass + at least every 72 hours (if I'm wrong, someone please correct). There'll be free wifi for sure at Frankfurt Airport.
Seat reservations are best done somewhere else than Eurail pass (booking fee there), especially for the countries you're travelling to.
Germany and Switzerland both have optional reservations.
- bahn.de : 4,50€ per journey (no matter if 1-2-3 trains)
- tickets.oebb.at : 3€ optional seat reservation, mandatory (Italy) : 3€ Intercity trains, 10€ Frecciarossa trains
The Austrian Railways website (ÖBB) can make seat reservations for Germany, Austria, Italy + cross-border trains to Switzerland (and some other countries).
Seat reservations in Germany are a good idea on busy mainlines (Berlin - Frankfurt - Switzerland for example), esp. on Fridays or holidays.
Reservations are a waste of money in Switzerland. Nobody does them, trains are frequent and very rarely full.
Let us know for any other questions.