I tried to install UCS second node as backup domain controller in a new servers vlan/subnet but the domain join part of the install failed. I used the manual network setup option and set the existing UCS server as DNS server during install. I also checked the firewall logs and there was a only a single ssh connection to the existing ucs server from the second node ip. I also could not find any clues from the existing UCS server logs. When I retried the backup domain controller install in the same subnet as existing UCS node, the domain join worked.
Please add support for installing UCS backup nodes on a separate subnets.