How do I import/merge one lightroom classic catalog into another when the photos are on Synology NAS

I want to merge a small catalog into my main lightroom classic catalog. The photos are all on my Synology NAS, in different folders, and the catalogs are on my local hardrive/computer. I cannot get LRC to recognize the secondart catalog photo folder(s) when I click "import from another catalog."  How do I accomplish this?