As long as both your iPhones are using iCloud Photos, the syncing will keep the photos on both devices identical. Anything you do in Photos on one of the synced devices will be synced to the other devices as well. You are always working with the photos in iCloud and on all devices at the same time, independent of the device you are currently using to access the photos. You must not delete any photos from your old iPhone that you want to keep on the new iPhone.
I hope you have not yet emptied the Recently Deleted album on any of your iPhones. Then you can recover the deleted photos by selecting them in Recently Deleted and putting them back. The Recently Deleted album is the only safety line offered by Photos.
If Recently Deleted is already empty you have to fall back on the backups of your photos you made before you deleted them.
Did you make a backup on a computer, before you set up the new iPhone? You need a backup that contains the photos. After you enabled iCloud Photos the photos are no longer backed up by the iCloud backup of your iPhone. And even a backup on a computer can only include the photos that have been downloaded from iCloud to your iPhone.
If you have a backup on a computer that you made, while the photos were still there, try to restore the old iPhone from this backup, while iCloud is disabled: Restore your iPhone, iPad, or iPod touch from a backup - Apple Support