You get via ÖBB a PDF and you have to print it (this say the website before booking, I never got reservations for Italy before via ÖBB).
I see that you are from Belgium, you can also ask SNCB/NMBS International Ticket Desk in Brussel South if they can sell reservations for Italy. If you are travelling now, you can also book at DB, SBB CFF FFS, ÖBB or Trenitalia ticket offices the reservation.
You will never get reservations in the app, also when you buy it on the app, because the app only shows the Interrail Website. Website and App are two different systems and don’t work together. On the app you have your ticket (the Interrail pass) and on trains were you need reservations, you have to show also the reservation.
In your case I would use the regional trains, because the time difference between long distance trains and regional trains is minimal on this route (but on regional trains you don’t have a selected seat and if the train is full you have no seat). Or you choose a Intercity train, this will cost only 3€ for the reservation.