The DocumentStructureAccessAppThing is designed to be used for the WTDocument use cases.
If you want to get the EPM Document (CAD Document) structure you need to use a different approach.
The connector that you can look for in the OOTB services is "ptc-windchill-OData-connector".
For Example the call "CAD_Get_Structure_Internal" can be used to get a CAD Structure from Windchill.
But as Andra wrote, sharing a bit more details about your environemnt and detailled use case would be helpful.
Depending on your Navigate version and your use case you would have to use a different approach.
The below example is using the CAD_Get_Structure_Internal service with an ID (EPM oid) and the $expand of "CADDocument,Components($expand=CADDocument,Components;$levels=3)". Which would get the epm structure of the assembly for 3 levels)
