Hello,
First you need to make sure all photo files are in a supported format. The Photos app supports common image formats such as JPEG, PNG, GIF, BMP, TIFF. And try mapping a shared folder on the NAS as a network drive. This can be done via "This PC" > "Map network drive". Then, open the Photos app and check if it recognizes photos from this newly mapped drive. If the problem still cannot be solved, you can consider using other third-party image viewing software, such as IrfanView, XnView MP.
Best Regards
Zunhui