If you look around at other posts on the forum, you will see any number of issues with Cox and their recent implementation of IPv6 into their service. It is not hard to see that Cox users are having a number of issues......while users with Time Warner and Comcast are not. This points to Cox doing something different in their IPv6 implementation, but I'm not on Cox, so I can't provide details on that.
Cox seems to be telling users that the AirPorts are not compatible with their service....and has even posted a notice on their website to that effect.
You can disable the IPv6 Internet connection that is being delivered by Cox by changing a setting on the AirPort router. This might help temporarily, but unless Cox is implementing more changes soon, you won't be able to realize the benefit of IPv6 from Cox, since you won't be using the service that you are paying for.
As far as I can tell at this time, Cox is telling user that they will need to use another brand of router if they want to use the IPv6 connection. You might want to discuss that with Cox.
Are you using a Mac with a recent operating system installed on the device like El Capitan, Yosemite, Mavericks, or Mountain Lion?
The SB6190 is a new modem, so we don't know much about how that product is behaving. Another popular model, the SB6141 is well known as not being that co-operrative with Apple routers....or vice versa depending on your view. Here's just a sampling of that model on a quick forum search:
https://discussions.apple.com/search?q=sb6141