Cannot join domain


I would like to install UCS slave in our branch office. The IP subnet is different from UCS master.
After few tentatives to join domain though the ucs portal I had this error message :

‘’ Join error
The system join process could not be completed:
Please visit for common problems during the join and how to fix them – The ssh-login to failed with " ". Please make sure the account root exists and is a member of the Domain Admins group!
More details can be found in the log file /var/log/univention/join.log .’’
In join.log I have same message…

I did ucr get server/role and i get : domaincontroller_slave
Log in SSH on this server I tried to do ssh to the master UCS and connection close. (works from another computer on the same network)
It seems that I cannot do any ssh command from this computer even if I 'm root.

Could you please help me with that?