Your web server is an internal one isn't it?
Client is also on same network (LAN) correct?
Why would you want to access this web server from client going though Zentyal server? Except if you have some specific network constraint, there is not strong reason for such design.
Then, remaining question (that has been asked less than 2 days ago BTW in same forum, feel free to use search feature
) is:
"how to reach this server using external domain name?"
Quite easy: just define in Zentyal DSN section you external domain and maintain internal hosts (with internal IP address of course)