On eurail.com (2€ booking fee per person per train) or somewhere else with less fee :

https://www.seat61.com/interrail-and-eurail-reservations.htm

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

Keep in mind that seat reservations and the app/pass are completely separate so it doesn't matter where you make them. Whenever a ticket inspector comes you show them both the pass (app) and the seat reservation when required (PDF or printed)

So as long as I have a QR code on my app and the paper seat res Im good?

Yes, but you don't need the reservation on paper unless it's a real for a sleeper or couchette on a night train.