The manual says that it is only achieved with Displayport. Funnily enough, I also get it to work via the IcyBox USB-C docking station with HDMI.

It doesn't work for me via HDMI either. However, I achieve the maximum resolution with Displayport. The user manual says that the maximum resolution can be achieved with both connections. It can't be the graphics card, because I can set up to 4k on another monitor. This also causes a black bar to appear at the top and bottom of PbP. I contacted Philips directly yesterday and am now waiting for a response.

I have the resolution via Hdmi, so it's probably the graphics card! Rx570 works tip top.

It didn't work for me via HDMI either, only via DisplayPort. Both cables were included in the delivery.