You don't need the app for reservations, although using it should be possible as far as I know. Reservations are completely separate from the pass.
For international TGVs from/to France, there is always a risk that the quota for Interrail (a French speciality) is full.
You should also be able to use the Interrail website, although I see that the Paris - Luxembourg TGVs never have a price (not even on a Wednesday end of January), so it looks like there's some kind of problem; I just sent a bug report to Interrail about this. Another option is to use a domestic TGV Paris - Metz and change to a regional train to Luxembourg.
For Paris - Frankfurt, the Interrail website does give prices (13 EUR reservation + 2 EUR booking fee, electronic ticket).
You could also try booking by phone via Belgian railways. They can book both of these trains. For Interrail reservations, they don't charge a booking fee but you'll have to pay for the phone call of course. They will send reservations via email if available in electronic format and they don't need any pass number.
Thanks for the idea about phoning the Belgian railways!
According to the app and the website I cannot use the Eurorail/Interrail booking tool since I didn't my my pass directly from the official site, but from a local distributor. That apparently makes it impossible to use that tool. I could use the online tool if I had a paper pass but I don’t there were only mobile passes available. I still have my pass cover number from my purschase mail so I can't really see why Interrail wouldn't let me use the online booking tool.
You should be able to use the Interrail reservation service with any Interrail pass bought anywhere:
https://www.interrail.eu/en/plan-your-trip/reservations/get-started-reservations
I haven't booked anything with it though because they charge extra fees. But I also have the comfort of living close to a rail station where I can book many reservations.
Hi Johan, just to confirm, you should be able to book your reservations through our system - regardless of where you bought your Pass. Alternatively you can also book your reservations with the train carriers directly, like rvdborgt suggested.