If I understand this correctly, the problem the OP has is that he needs to QUERY the Zentyal DNS server from a different network other than that configured in the "internal" interface. Try this:
# vi /etc/zentyal/dns.conf
[...]
# Internal networks allowed to do recursive queries
# to Zentyal DNS caching server. Local networks are already
# allowed and this setting is intended to networks
# reachables through static routes.
# Example: intnets = 192.168.99.0/24,192.168.98.0/24
intnets = IP.Net.work.1/24,IP.Net.work.2/24
[...]
Then restart Zentyal DNS service to apply