How do I recover permanently deleted photos on iPhone?

Permanently deleted photos cannot be recovered from the device. The "Recently Deleted" collection is the only place where the iPhone is saving photos for recovery. When "Recently Deleted" is empty, the photos are gone.

Then you need to use your own the backups you made while you still had the photos. Why kind of backups are you keeping?

If you have other devices syncing with iCloud with your iPhone, for example a Mac, the Time Machine backup of this Mac might contain your photos. Also look at any places where you may have shared the photos, the attachments in Mail or messages.