I have built a UCD and elected not to auto discover or auto join domain after build.
When the OS loads, you are prompted to login. As expected.
Is there a default user account? The session does not permit logging in as root. I do not recall the installation process giving the option to create a local user.
I have tried "Administrator" with no password and "Administrator" with the root password that was assigned during the installation. No luck.
I built another UCD and elected to auto join domain, but it failed to find the domain controller (perhaps a network issue). So I have the same problem - not being able to log into the UCD after installation in order to join the domain, or simply use as a standalone without management.