Transferring photos from one iCloud account to another one

I recently made a new iCloud account for my phone, but when I merged it and switched over only about 1/5 of my photos came within. My entire photo library is on my old iCloud, and I can sign in on windows for example, but I want to be able to transfer all of those photos to my Phone on it’s new iCloud Account. What’s the easiest way to do this?