Correct - you need to turn face detection on in the camera. It will show a box around the head when it picks up a face. But I thought you had a NVR5208-8P-4KS2/SE. If it’s a 4-series and /L that would explain why it was so cheap.
But even a 5208 /L won‘t do the AI stuff - you need to see a capital I in the model name. Have you flashed your /L with /SE firmware? From what I recall you bought your gear from a friend who worked for a distributor. That person should be able to make it work. The problem with a lot of this stuff is that it all comes from China and it all comes in brown boxes with a little 2” square sticker on telling you what’s in the box and it’s very easy to buy stuff with a letter or two different that doesn’t work. I sold a system to someone a bit over a year ago and when I went to set it up it didn’t do the alerting because the warehouse had shipped the wrong NVR. They looked identical but I had to go back the following day to get it working. And because I’d unboxed it I couldn’t return it. Nasty!
Whoops, no idea why I thought I had a 4208!
From checking the software it's this:
DHI-NVR5208-8P-4KS2E
So your memory is infinitely better than mine!