A Photos Library on a network share is not supported by Apple, as far as I know. However, if you simply want to browse the images stored in folders, that would be different.
Not all NAS work with Nitro on iOS (Mac seems to be fine in general) – I have tested Synology (which worked) and UGreen (which did not work). The basic symptom is that the app can see the folders, but the contents are reported by the OS as empty. As far as I can tell, this is either a configuration issue or bug in the NAS (since some NAS work fine).