Xbox One S doesn't recognise 4K compatibility on a 4K projector

No, I don't have to. The projector automatically recognises 4K. There is no option to activate. I didn't have to for other devices like Apple TV 4K and Firestick 4K. They were automatically recognised with 4K HDR.

To verify, this is going directly to the device and not through a receiver or any other device like a splitter or switcher?

But their problem is that it's not recognizing that the display is 4k capable, not that it's not playing games in 4k. In the Display Settings it has the options for checking if your TV is 4k compatible, and that is showing the display is not capable when
it is. Debating what can and cannot play in 4k via the One S is not the issue.

@KayKay Lavan thanks for posting the make and model. I'll search and see if there is anything I can find for that, or dig up some other troubleshooting steps.

I don't think so, Robin. I had an LG OLED 4K TV. And the Xbox One S was connected to to the LG TV and I used to play 4K games. Even if that's the case to play only 4K Blu Rays, it's still not supporting.

The Xbox One S plays games at 1080p but if played on a 4K screen, it upscales it to 4K,

The One X is needed for true native 4K,

I upgraded my Xbox One to the One S at launch to get full use out the 4K media content,

Then i upgraded my One S to the One X at launch to play games and media at true 4K,

seeing that you've already tried what i would have suggested, I'm not sure of what else you can try.

I have tried the cable supplied with Xbox and other expensive HDMI 2.0a cables. No, I am not using a switch. It is directly connected to the Projector's HDMI port. The projector has 3 HDMI 2.0a ports. I have tried all the 3 ports.