Because Circle utilizes ARP Spoofing to manage devices (see more here on How Circle Works), the DNS settings on our device may fall back to the router settings.
What is DNS?
DNS, or the Domain Name System, is like a contact list for the Internet. When you make a call you're actually using a series of numbers to connect to the right line. A contact list equates that number with a person. The Domain Name System does the same thing for websites and servers. While you enter Google.com to open your search engine, your browser and computer connect to a server identified by an IP address (a series of numbers seperated by periods) to load that. The Domain Name System connects a domain like Google.com to an IP address.
Some techy folks out there like to set up custom DNS settings on a device for different reasons. For example they might be using a parental control service to manage their DNS settings and what the device can and cannot connect to. You might even say that Circle Home is like custom DNS settings that you set up differently for your devices depending on their Profile and Filter Level.
If you're using custom DNS settings on a device you probably know about them already. Importantly, those DNS settings won't stick if the device is managed by Circle.
Why can't I use custom DNS settings on my device?
When you manage a device using Circle (set it to a profile with a Filter Level of None, or Adult and below) the device's outbound Internet traffic will be intercepted using ARP spoofing. If the traffic is allowed through based off your Filter Level settings its DNS will fallback to the router's settings and not the device's. There isn't a way to prevent this other than setting the device to "Unmanaged". You'll need to make the choice between those custom DNS settings different than your router versus managing a device using Circle.