iCloud Photos

The idea is that you must have a Mac somewhere with those photos on it. At some point, you synced them with your phone. If you turn on iCloud Photos on your Mac, those photos will be synced to iCloud. Then, when you turn on iCloud Photos on your phone, they will come back via iCloud.

My recommendation would be to connect your phone with your Mac one last time and make sure you have all of your photos on your Mac. Then you can back them up and/or archive them.

It means that you cannot sync to the iPhone and use iCloud Photos at the same time, it's one or the other. Firstly, make sure the photos are indeed on the Mac, and if so, then you can enable iCloud Photos on your Mac and they will upload. After, you can turn on iCloud Photos on your Phone, deleting the Finder-synced photos, and all your photos will then be available on all devices.

Learned from etresoft

Edit: etresoft your typing is too fast for me. LOL