You don't need a mobile Interrail to book a reservation through the app.
You don't need to book reservation through he app. I have never used the app (or the Interrail website) to book reservations because of the extra fees they charge (2 EUR per person and train and 9 EUR per order on top of that for reservations that need to be sent via the post).
That being said: you can book reservations for domestic TGVs if you go (in the app) to More > Seat reservations > France > I have a paper pass > Book TGV trains with SNCB.
This will cost an extra 4 EUR per order (which can include multiple people and multiple trains) but it does work via the app.
International TGVs can currently only be booked at the station.
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