> The HDMI port on the computer side is not required.
> Fortunately it has a DisplayPort output and I use a cheap DisplayPort --> HDMI cable
Well, what you are essentially saying is "no and yes".
An HDMI cable must be connected from the LG Portrait to the computer. But if you don't have an HDMI port on your computer, you can use an HDMI adapter. In your case, you happen to have a Display Port on your computer. Otherwise there needs to be another HDMI adapter to a USB-C port or USB-3 port.