Face recognition in photos on iphone

This does not work for me. If I add a new photo of a face I have already identified then the phone recognises that face. However if I add a photo of someone who I have not previously identified then the procedure you suggest takes me to a section called Related and tells me no related found. My problem is I cannot assign a name to any new face, and, as I have older photos where I have assigned names, there must have been a time when I could do this. What I need to know is how to assign an ID to a new photo

As a test I added two photos at the same time. One of these had 3 people in and 2 of those had been identified in earlier photos. The third person had never been identified by me. The second photo only contained an image if this third person. The phone identified the two people in the first photo who I had previously assigned names to, but failed to identify the third person, and did not give me a thumbnail of this image to add a name. For the second photo the phone found no related image and also failed to give me a thumbnail of the face. As the phone recognised 2 of the people in the first photo then the phone must have had timevtonrunnface recognition.

These photos are held locally on my phone. I hope this helps to identify a solution

I also would like to know how I originally assigned names to faces on my earlier photos

I am usin iOS 12.1.2. I will have a look at the link you’ve included for iOS 11 and later.