Same topic has been asked here …. times. can only be done via DB Reisecenter-or any other counter that is linked to this, as these are all global price trains-and this means it also needs a competent worker-not all know how to handle it.

Plan B: can book online seat (but probably only 30 days advance) on PKP-intercity from Rzepin=1st stop in PL just over border/river. take local train before to there, but double check tines.

Do not worry too much-no these trains are not ´full´ weeks advance.

But short in advance these trains are often fully booked. I had to experience this by myself once. They are not super long and there is not only international but also Polish domestic demand for this trains…

so definitely book a domestic reservation and then get the Berlin-Rzepin one in Germany.

Since I am from Denmark where everything normally is a breeze to get fixed, I found togrejse.dk where you can request seat reservations for a small fee of 30 dkk. Finally a site that's easy to use.