You can attempt to recover the photos from iCloud. Log into this website ➔ www.icloud.com with your Apple ID and password that was used on the phone that had all the photos backed up to iCloud. Once logged in then check the Photos category on iCloud and see whether you see any photos there. If you do then those are the photos that you can sync with your found iPhone by signing in on your found iPhone with the same working Apple ID and turning on iCloud for photos (Settings ➔ Apple ID ➔ iCloud ➔ Photos: Ensure that this is set to ON). If there are no photos found then, of course, you cannot use the iCloud method to retrieve the photos.
The other method to recover photos, if the method above does not work or is not applicable, then you will have to restore your iPhone from your found iPhone's backup file that you may have created on your computer. This file had to have been created before it was lost and when it had all the pictures. If you do not have this backup file, from that far back, then this method will also not work.
If none of the above works then you have lost those photos permanently.
Axel F.