We confirmed with several hotels in walking distance that they have rooms available. You can book directly via their websites:
Hotel 3 Könige: https://www.h3k-aachen.de/de/?utm_source=gmb
Poha: https://pohahouse.com/
Hotel Baccara: https://www.hotel-baccara.de/
Motel One: https://www.motel-one.com/de/hotels/aachen/hotel-aachen
Aachen Tourist Service also negotiated rates for DW attendees in selected hotels in the city center (Motel One, Ibis Marschiertor, and B&B Aachen City). You can book these via this link:
https://tportal.tomas.travel/aachenkongresse/messe/quicksearch/TUR00020110521236863?lang=en&reset=1