The current implementation does not support stacked NVRs, which is a feature release by UniFi 2 years ago.
All cameras attached to the stacked NVR are currently not been seen by the integration.
I wanted the same feature, but during my attempts to get it working, I changed the protect local user password for HA and broke the configuration. After some time playing around to try to re-auth, I gave up and deleted the integration from HA and then re-added it. All the cameras from the stacked device turned up … YMMV