Please read the FAQ about the mobile pass.
Mobile Pass Frequently Asked Questions (interrail.eu)
Your mobile Pass is periodically checked against our system to ensure its validity and prevent fraud, which means that you need to be online at least once every 3 days.
We’ll send you a notification 24 hours before your 3 days are up so you can find an internet connection. If you are on a train, there may be on-board Wi-Fi that you can use. The app does not update in the background, so you’ll need to open the app when you find a connection so we can check your Pass.
If you haven’t been online in more than 3 days, your mobile Pass will be considered inactive and you will not be able to use your Pass until you connect to the internet and open the app.
the Reservation will not come in the app, but they will send you via email as a PDF, most of the time you can show it on the phone, sometimes print them or also like TGV/AVE France-Barcelona they come with postal mail at home.
Please try to book only essential things at Interrail.eu, because it doesn’t work well. Many reservations you can buy online (search here in the forum about Tips or ask here), many reservations can be done at SNCB/NMBS ticket offices or other like DB, ÖBB or SBB CFF FFS.