Article 12 of the Digital Services Act: write to the address above. It reaches a person, not an automated system, and it is answered in German or English.
Article 11 of the Digital Services Act: Member State authorities, the European Commission and the European Board for Digital Services can reach this service at the same address, in German or English. The Digital Services Coordinator for Germany is the Bundesnetzagentur.
This service is not obliged to take part in dispute resolution proceedings before a consumer arbitration board, and does not do so.