univention-join command is only used to join Univention servers into a Univention domain.
You seem to be a bit confused about the two different meanings of “domain” in the context of Univention servers. There are actually two different domain concepts at work: the Univention domain and the Active Directory domain. Unfortunately both use similar terms which leads to much confusion.
In the Univention domain context there are four server types: DC Master, DC Backup, DC Slave and member server. Each Univention domain consists of exactly one DC Master and any number of additional servers of any of the other three types. The command
univention-join is used to join a DC Backup, DC Slave or memberserver to a Univention domain’s DC Master.
It is not used to join such a server to an Active Directory domain.
Each of these servers can but does not have to be a member of an Active Directory domain, too. If you want to join such a server to an Active Directory domain, then you need the app “Univention AD Connector”. Its configuration is usually done via the web frontend. Read more about that in the documentation. In such a setup you still have a UCS domain — in addition to the Active Directory domain. However, this also means that you need a UCS DC Master. Simply setting up a UCS DC Backup without a UCS DC Master won’t work.
Now for the most important part: a UCS-based server cannot be an Active Directory domain controller together with Windows-based AD domain controllers! Therefore what you originally tried to achieve is not really possible (or at least not supported) at the moment. If you have an AD with Windows-based domain controllers, then a UCS server can only act as an AD member server without domain controller properties.