Where have you started your troubleshooting?
1. Test DNS lookup from the MX appliance status / tools page
a) if this doesn't work, then you have some issues on the uplink.
b) if this work, continue to the access layer
2. As NadiaK says, verify your DNS settings on the clients subnet on MX Configure / DHCP page.
3. Verify your DNS settings on the wireless network if your clients are connected wireless
4. Verify your firewall settings for the specific IP address of the DNS server and that DNS are allowed (UDP 53).
5. Try update your MX to the 14.53 version which is the stable release.