Fairly common issue. Some users store their catalog on a NAS, or a server share, or the cloud. Some just store the original photos on a NAS, server, or cloud. It works for some period of time. Then it fails big time.
Adobe does not support, nor recommend (in fact states to not do it) placing the catalog on a network share.
As for the photos, a bit of a debate on that, but not recommended. The link above does in fact state that you can, although appears to have a caveat of using smart previews. Eventually something goes wrong in between the NAS and the computers OS and LrC thinks the images went missing.
Some time ago this stopped working properly. The files went onto the NAS and could be seen in the folder via windows. For a while I got round this by dragging the files straight to the NAS then synchronising the folder. Now this doesn't work. The import may take in a few pictures to the catalogue then appears to do nothing. They are on the NAS but do not appear in the catalogue.
The most likely explanation is that the photos are already in the catalog, that's why Synchronize Folders doesn't help. If the photos are already in the catalog, it is a bad idea to import the photos or synchronize the folder again. You have most likely (accidentally) imported the photos into LrC using Move or Copy, and so now LrC thinks the photos are not on the NAS while you think they are on the NAS.
To figure out what to do next, you have to find the photos in the LrC Catalog. Please follow all four of these steps in order:
1. In the Lightroom Classic Library Module, on the left, under Catalog, click on All Photographs 2. Turn off all filters (Ctrl-L once or twice) 3. Turn off all stacking (Photo->Stacking->Expand All Stacks) 4. Search for at least one of these photos by file name using the Lightroom Classic Filter Bar