Please help me claw back some compensation money from Deutsche Bahn from abroad

So long story short DB owes me some €300 (cancelled trains and overnight stranding). It's been 2 months and all that has happened so far is this:

• NS International (Netherlands), the ticket seller, refuses to compensate, refers me to DB (the operator) (I live in the Netherlands, so I buy tickets from them)
• DB has informed me the only way to do this is to send in a paper claim with all the evidence
• I send this via registered post, but it comes back 3 days later as undeliverable, because the address DB provides everywhere is:

DB Dialog GmbH, Servicecenter Fahrgastrechte, 60647 Frankfurt am Main, Germany.

Notice how there is no street name or number. I can't even try another provider like DHL because they won't make a label without the street name + number in Germany. In fact DHL won't even recognize 60647 as a German postal code! I literally cannot find a physical postal address to send something to. This must work in Germany, but how can I get it to work outside? I'd really prefer a shipping label, because I know DB is going to reject the claim, and I will have to go further… so I just want to make sure all my paperwork is tracked.

I called Deutsche Bahn and they hung up on me because I don't speak German. So now I don't know what to do other than spend a whole day taking a train to the nearest German city to drop this off at a train station where they will apparently take care of it.

Sometimes I see posts on reddit of travelers unable to claim their compensation because they don't have an IBAN. But I do have one, I just can't seem to post the letter.