Have you previously been using iCloud Photos (Library) on your Mac?
Then check, if iCloud Photos is still enabled in the System Preferences > iCloud > Photos > Options, and also in Photos > Preferences > iCloud.
If iCloud Photos is enabled, but the photos do not sync, have a look at the status line in Photos for Mac, as described here: Check the status of your library in Photos for macOS - Apple Support
As long as Photos is still stuck on uploading photos to iCloud, it will not download newer photos from iCloud. In that case you have to identify the problematic items, that will not upload and remove them from your Photos Library.