Correct way to query Firewalls assigned to a Tenant

Hi all,

I am trying to find a way to get all firewalls of our Partner-Account and whom they are assigned to. I try to get the informations via API that are available on the Sophos Central Web Interface in the section "Sophos Central – Firewalls" -> "Lizenzen & Abrechnungen" (licenses and billing):

I found the Endpoint "api-{dataRegion}.central.sophos.com/.../firewalls" which provides firewall-informations for each tenant. In many other cases informations from this endpoint match the infos displayed in the WebUI, but for the specific example provided in the screenshot it does not. There the endpoint simply returns an empty result.

Is the endpoint the correct way? Or is there any other possibility to retrieve informations of firewall-assignments via the REST-API?