Storing Photos and External Drive and Syncing With iCloud

I recently had to move all of my photos and videos from my system photos to an external drive because I have around 150gb that was filling up my Macbook Pro. I was able to get the System Library switched over to the external drive but when I try to reconnect iCloud by going to Photos > Preferences > iCloud it gives me no options and reads, "iCloud features for Photos is only available in the System Photo Library."

I want to be able to keep my photos stored on an external drive but still have iCloud sync my photos whenever the drive is connected.