I believe the config for network interfaces via the gui and also OMV-First aid is bugged as it throws a validation error if you try to submit it with no nameservers or search domain. Yet it's perfectly valid to have an interface with no DNS particularly if it's a second interface
Omv-firstaid will also throw an error "ERROR: dnsnameservers: The value '' doesn't match the pattern '/^(\S+[,;])*\S+$/i' if you set the interface to use DHCP even though it never actually asks for a nameserver
Also if you do try and apply a config with eth0 as dhcp and then eth1 as static (with no default GW, as that interface is purely to reach a certain subnet without going through he router) it will just break networking when you apply it by removing the config from /etc/network/interfaces