Trying to change DNS settings, will not apply or save them

Hello all,

I'm trying to change DNS settings (generally for spoof location streaming) in the C6300, and I have done this on other systems with different modems. When I try to change DNS for a particular device (let's say fire tv in this case) and it doesn't work, a lot of the DNS services say that ipv6 needs to be disabled for it to take place (change the DNS for the router/modem not just the end device, fire tv box). When I go into routerlogin or my IP address via browser, I can never get the DNS settings to change to the numbers I desire. I click apply and the screen always changes to "invalid IP address", new DNS settings never take place. What's more, my device has a DNS that ends in ::feed, which seemingly never changes. I've included screenshots of the internet setup screen where one theoretically can change the DNS, I try to change the numbers that you see, but the error screen pops up. Is it an IP issue with my ISP (Comcast)? Thanks in advance.