The combination modem/router is one of the devices supplied by Comcast's Internet service. Due to the custom firmware, the device does not permit any changes to the DNS server settings. This is based from the support article for this particular model:
What's the next step?
There are a few options to explore:
- Contact Comcast or SMC support to confirm if your device supports changes made to the DNS server settings. There's a possibility that Comcast may be able to release the device from its restrictions upon request.
- If Comcast or SMC cannot assist you with the device, your computers can be configured using the steps at https://support.opendns.com/forums/21618384-Computer-Configuration. This is as effective as using your router, but you'll need to set up your computer's manually with the above instruction set.
I still want to use a router.
This is one of the most requested options for configuring OpenDNS, because it can filter all computers and smart devices without modifying each setting individually. The alternative to the above options is to set up a second router to your network, and configure that device with our steps at https://support.opendns.com/categories/20080020-OpenDNS-Device-Configuration. This will act as the gateway for your home, and all the requested domains will be routed to OpenDNS.
No better security, then physical security.
When proceeding with the option for installing a second router, we usually recommend changing some of the WiFi settings so it's not accessible. However, there does not seem to be any information in the provided document for changing this feature. Usually entering the router's configuration page by typing the gateway IP address into the web browser, may help you gain access to the WiFi settings.
If changing the WiFi password is not available, then the best choice would be to lock the router device(s) into an area where only the admin/parent will have access to. This will prevent unauthorized access to network settings to circumvent your OpenDNS filters. Comcast may have some support for customizing the router's WIFi settings.