What a drama! I found no way to assign the Windows 10 VM host a fixed IP address inside the QNAP Virtualisation Station. I was excercising the virtual switch and software switch settings up and down – to no avail. Finally I found the reason of this treason: Since the Windows VM was migrated from Parallels Desktop (via Virtual Box) it still had two other network cards in its belly. Of course, they were hidden in the Device Manager. But as they had this one fixed IP address already configured, it was not possible to add it to the new VirtIO interface. Once I had made these two other network cards visible, I deleted them and – voilá – the fixed IP address now could be assigned to the VirIO interface without any trouble. Before, only DHCP would work. Maybe this information helps someone who is also trying to get a VM with fixed IP addresses set up in QNAP virtualisation station.