external photos source will not sync mac to iphone

I currently have an external hard drive for my photos. they are all organized by album for ease of syncing. the computer recognizes all devices and locations but when i tap sync nothing transfers. I've tried creating a new photo library and that worked but when i switch back to my original it continues with the same problems. I have contacted apple support with all the screen sharing information and problem solving issues and i continue to have the issues with syncing. Anyone else have this problem? Any recommendations for a fix? I do not use iCloud, everything is already on the computer but want to be able to put certain albums on the iPhone. Thank you for reading and hope to get this resolved.