The Tomato based routers have a simply way of redirecting traffic sent to Google DNS (or any other DNS for that matter). Check the checkbox Intercept DNS port 53 and all DNS traffic will be sent to the DNS servers you have specified on your router. No longer will devices such as Chromecast and Roku be able to communicate with Google DNS. All DNS traffic on standard port 53 is intercepted and sent to our servers.
First step of troubleshooting should always be to restart device(s) and router. Then check your setup status in your SmartDNS dashboard.