Yes sure, they're valid on all trains on that route. Seat reservations aren't a thing. Simply log the train to the app before boarding and hop on.

I bought a mobile global Eurail pass but after reading the reviews of Rail planner app.  I am worried…so many bad reviews about the app. And I am having problems to get reservations.
Now I want to get a paper Eurail pass and reserve seats at the train station manually. 

How can I change mobile to paper pass? I got my pass with the current promo price and bought a plus to insure the pass. 
Thank you in advance.

The current promo is not available as a paper pass. You can exchange but you'll have to pay the price difference on top of the €15 exchange fee.

However, the app seems to have matured over the past year and I hardly see any error reports anymore.

In addition, changing to a paper pass won't make any difference for getting reservations. That is a completely separate process. Please take a look here:

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

If that doesn't help, then please mention route, date and departure time for each train you can't book reservations for.