How do I recover permanently deleted photos on iPhone?

You don't recover permanently delete photos. That is what it means to permanently delete a photo, you would have had to save it in another location yourself.

When deleting a photo, it will remain in the Recently Deleted folder for 30 days in case you change your mind. If you delete from the Recently Deleted folder before that time, you are met with a warning that continuing with that action will result in the photos being permanently deleted.