It's on the latest firmware as far as I know and I double check every do often to make sure it hasn't missed one. They say it won't show HDR just automatically adjust when it detects a HDR signal coming in.Many of us have had success using Monoprice certified premium HDMI cables with the X800. They are pretty cheap and they get them to you fast. Might be worth a try. Is your TV on the latest firmware? Late last year the Hisense needed a firmware update in order to get HDR over HDMI inputs 3 & 4. If you bought your set recently it probably has it already. Don't know where you see notification that it is in HDR mode. On a lot of TVs you see a brief popup that tells you the TV has sensed HDR and is in that mode.
It is also known it doesn't hit all the specs but gets it to about 83 percent of what a fully compliant set can hit. I guess many wont do the maximum brightness anyways but this thing seems way brighter than my old Sony 50EX645 I was perfectly happy with. It's only around 550 anyways so I think most buyers are happy with it considering the bang per buck you get.
The eBay seller has said he is going to send me some different supposedly compliant cables and the one I'm using now shows HDR on the Sony when it's playing a HDR disk and the bit rate hits around 50 at times. If the new ones don't work I'll check out mono price as I've bought cables and speakers and switches before from them and they've always been high quality for a great price.
I've had issues before with almost any hdmi cable and certain gear before so not surprised my combo might be picky.