Please call ENTOUR the norwegian Ticket agency they can issue the reservations as PDF document.
This way you even save the service charge of Interrail

I have been now on the phone with them for 15 minutes and no answer… plus, as the phone is Norwegian, it does cost a lot to call them

If you wanna reserve in advance there is currently no other way when Interrail fails :/

Thanks for your help @seewulf. I finally managed to reserve the ticket with ENTOUR, I just needed to call in better days/hours (called on a monday at 8.00 am and I was attended pretty fast). In case anyone needs it, I bought a 5 euros credit with Rebtel, so I could call with no extra costs to Norway. 

@seewulf, do you happen to know if I have to do anything more to link the reservations to the interrail pass? Or shall I just show the reservation AND the interrail pass when boarding? Sorry, my first time using interrail.

When boarding you show nothing. It works like in any train=not plane. There are in general NO access checks. There is thus also noone to help you or point the way-you are big enough to do that all by yourself.

Whilst you have found your allocated seat (look first for car nr and when In car for seat nr) sit down and stow the overlaod on luggage any passholder seems to have above you in rack=not beside you.

Whilst train is on the move a conductor will pass by and check the tickets-show pass (activated trip and if s/he asks, also the REServ (probably not if you sit in right seat) and thats it. On very, very long trips after a whoile a new conductor may pass by and ask again-but I somehow remember they have a fairly efficient system in NO that they can even track this.

Works like that about anywhere. In trains with No or optional REServ-this conductor is the only one for checks-if s/he even comes.

You still have to log your journey´s in the Railplanner app on the train you show your Interrailpass with the logged train and the reservation.

Thank you @mcadv, that was clear.

Thank you also @seewulf. To log the journey do you happen to know if I need to do it much in advance? I am asking just because I do not want to activate the pass a lot before (in case my phone gets stolen or breaks). Since I have the reservation, I suppose it does not matter a lot, but this pass has given me lots of surprises.

Thanks!=)

No do it last minute when you are sure you catch the train or do it directly after boarding.
As you never know what can happen before you are on the train :) 
Feel ill and extend your stay. Bus or Taxi get stucked in traffic.