Unable to reserve a cross-border seat (Poland-Germany & Germany-France) via DB

Hi!
I'm trying to reserve a seat/seats on a Warsaw - Paris journey, via Berlin & Frankfurt.

I can see the whole journey and each leg, but DB won't let me reserve a seat directly on the website.

The issue seems to be the Warsaw - Berlin and Frankfurt - Paris legs, ie crossing the border.

If I book separately, it seems that I can reserve the Berlin - Frankfurt leg with no issue (this, paradoxically, is the leg that the 'site refers to as 'high demand') - the other two legs aren't currently that busy.

Changing the date, class, changover time... makes absolutely no difference.

I had a similar problem booking Warsaw - Rijeka and found that Warsaw - Pszczyna, then Pszczyna - Wien (same train) got around the problem, but no luck here.

On the Interrail Rail Planner it specifies that reservation is required for precisely the legs I can't book, but not the one I can(!)

Any tips on how to get these legs of the journey reserved?

I’ve been told that I may have to go to the international ticket office here in Warsaw to reserve the seat for the Warsaw - Berlin leg, but no idea what to do about the Frankfurt - Paris leg.

Thanks in advance!