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This combination router and modem is typically used as a bundled device with a service provider. Most, if not all, service providers use a custom firmware program that prevents users from changing the DNS server settings on the device. This is based off the support article below, which only provides steps for changing the DNS settings through your computer only.
So what's the next step?
There are a few options to explore:
1. Contact your service provider or Motorola support to confirm if your specific device supports changes made to the DNS server settings. There's a possibility that your service provider may be able to release the device from its restrictions upon request.
2. 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 my 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.
RECOMMENDED: To make sure that the second router is the only device users can connect to, it's recommended to change the WiFi password and hide the WIFi name from being broadcasted. Unfortunately, the manual above doesn't offer steps for configuring the WiFi, however you may be able to change the password within the configuration page.