During 802.1X authentication, the username presented is not the device’s MAC address.
However, we need to retrieve custom attributes from the ClearPass Device Guest module, where devices are indexed by MAC address.
Add a custom filter to the Guest Device Repository Source so that device records can be correctly matched and the required attributes retrieved.
SELECT tgu.attributes->>'Custom_ISID' AS custom_isid FROM tips_guest_users as tgu WHERE tgu.guest_type = 'DEVICE' AND tgu.user_id = '%{Connection:Client-Mac-Address-Hyphen}'