Enroll device without GMS Setup Wizard
what Device is it Zebra, Honeywell, Datalogic, ...?
it's this one: https://www.fantec.de/produkte/heimkino/alle-media-player/produkt/details/artikel/2466_fantec_4ks7000/
We want to use it as a digital signage player.
In my experience with Digital Signage products they typically have a CMS and their own form of lightweight MDM for managing the devices. Perhaps thats what the customer setup wizard was supposed to be for? I don't think you're going to be able to enroll them under Fully Managed without an OEM specific mechanism for doing so. For example, you can enroll AOSP / NON-GMS versions of Zebra or Honeywell devices into SOTI as Fully Managed Device Owner devices using the staging mechanisms that they expose. Without the OEM specific option you'd be stuck using one of GMS setup wizard options (DPC Identifier, NFC, QR, ZTE), which it sounds like are not available to you. Check with the manufacturer to see if they offer a mechanism.