Where do the clients get the DNS server IP address from? Is the router running DHCP and giving the DNS server or are the clients statically configured?
What's the problem you see on the clients? Timeout? Do you see a different result using the default DNS server vs. specifying the DNS server. For example:
smf22@lampu:~$ nslookup ubnt.com Server: 8.8.4.4 Address: 8.8.4.4#53 Non-authoritative answer: Name: ubnt.com Address: 52.10.164.58 Name: ubnt.com Address: 52.35.255.243 Name: ubnt.com Address: 52.38.61.152 smf22@lampu:~$ nslookup ubnt.com 208.67.222.222 Server: 208.67.222.222 Address: 208.67.222.222#53 Non-authoritative answer: Name: ubnt.com Address: 52.35.255.243 Name: ubnt.com Address: 52.38.61.152 Name: ubnt.com Address: 52.10.164.58