I'm trying to set up an domain in Azure AD Connect on a Workgroup computer. The challenge that we have is, we have a dedicated domain controller for the purpose of Azure AD Connect and the other domain controllers are in a non-routable network for AD Sync server (We chose not to install AD Connect on Domain Controller for various reasons). Sites & Services are configured properly but when we attempt to add the extra domain, we get the error "The Specified Domain does not exist or cannot be contacted."
Routes exists, firewall rules allow standard AD traffic along with dedicated AD Sync server's traffic as well. If I attempt to join the computer to the domain, I am prompted for credentials which I think would be successful if I click OK.
Finally, we ended up upgrading the AD Connect version which resolved the issue. Appears to be a glitch in that particular version we had.
Might be in the user name you're using: https://blog.kloud.com.au/2015/12/16/azure-ad-connect-the-specified-domain-does-not-exist-or-cannot-be-contacted-when-adding-an-untrusted-ad-forest/