I think I was not clear enough. My intention is to use only the DHCP server of the router (FritzBox). Therefore I have set it up so that it always assigns the same IP to the OMV virtual machine and also gives it the name 'server'. The router identifies the virtual machine by its MAC address.
It's set up the same way for my HTPC, but of course with different MAC address, different IP address and different name. There it works. The HTPC is also the host for the OMV guest.