Vostro 3425 with USB-C dock: display not working

We just purchased the Vostro 3425 and docking station. But the external display does nothing.

The dock seems to work fine, Dell K20A001. Keyboard and mouse work fine connected to the dock. But the display does nothing, the Vostro does not find a connected display.

Tried other docks, HDMI and DVI cables, updated BIOS and all drivers, windows updates, several reboots. Still no display found.

Any ideas?

Thanks

Richard.