Unfortunately, the device is described as follows:
The IB-DK4040-CPD has two different video interfaces that enable the simultaneous connection of HDMI and VGA display devices in mirror mode.
Kaele12 is right about the settings. But I don't think the icy box can transmit the signal separately to both monitors, only mirrored. The display configuration of the operating system doesn't help. Because as far as I can remember, when I tried it on my Mac, I could only display the image mirrored on both monitors. Which corresponds to the description.

You can connect a monitor to both outputs at the same time, the picture is then simply displayed in mirrored mode.