Kudos again to @Moritz_Bunkus
Question
How can we restrict access to /univention only to local networks?
Answer
To restrict access use Apache’s internal access control and create a file name /etc/apache2/conf-available/umc-access.conf
whith the following content:
<Location /univention/>
Require ip 192.168.0.0/24 # put your local network address range here
Require all denied
</Location>
Then enable this file and restart Apache:
a2enconf umc-access
systemctl restart apache2