Extended display monitor stuck on 640 x 480, setting greyed out

[Ultramince]

LAPTOP-2AJGT6K5
The laptop built-in display works fine.
I have been using a 40" 1920  x 1080 LCD TV as a monitor via a 2 way HDMI splitter box.
This was a legacy from a setup with an earlier laptop I used to use.
I don't need the splitter any more so I removed it, and plugged the HDMI cable directly into the monitor. But when I do that, the 40" monitor display is then stuck on 640 x 480 and the resolution setting is greyed-out. If I put the splitter back in, then it works properly again.
The TV works fine with other HDMI inputs, and is able to detect changes in resolution without trouble.

[NathanS20]

Update the graphics drivers to the latest version.

[Ultramince]

I've tried that; it says they're already the latest version.
This setup worked fine with an earlier HP laptop and a older Toshiba one.
For some reason, my current laptop communicates with the monitor just fine through the splitter box, but not when you plug it directly into the monitor's HDMI input. It looks like the laptop doesn't recognize the HDMI protocol of the monitor, and  so defaults to the lowest resolution.
The monitor is perfectly happy with 1920 x 1080 from either of my older laptops, or any other HDMI source I plug into it.
I can just use the splitter as a workaround if I have to.
This is apparently a very common issue with HP laptops; maybe this is a workaround others can use.