Airport Extreme not accessible but Internet is fine

I have an Airport Extreme 3TB router (Cortexiphan). Connected to it is a 4TB external HD (Hippocampus). The 3TB is devoted to Time Machine while the 4TB is a shared network drive.

To extend the range in the house, I added an Airport Express (Walternate).

Periodically, I find myself able to surf the Internet and even look through the Time Machine history but unable to see the 4TB shared drive nor the data portion of the 3TB hard drive.

The Macbook Pro 15" Retina acts like the Airport Extreme isn't on the network. Even Airport Utility shows it inaccessible with the message "Cortexiphan was previously part of your network. Check that it is still in range of your network and plugged into a power outlet. Forgetting this device removes it from this network."

If I powercycle the Airport Extreme (Cortexiphan), everything works as expected.

What could be going on?

Before powercycle:


After powercycle:

I think you nailed it. I had forgotten that to be able to use the Uverse apps on the iPhone to control the TV that they had to be "on the same network" so the trick was to put the Airport Extreme into Bridge mode and let the Uverse router/modem combination serve the DHCP.

I'll set the Airport Extreme back to NAT and see if that fixes the problem.

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Ok. That resulted in a "Double NAT" error. I'm going to have to remember what setting I changed on the U-verse router before I get a valid test.

We never use the app for controlling the tv and in a week and a half I'm switching back to Comcast and buying an ARRIS / Motorola SurfBoard SB6183 modem. I'll just have to remember that the Airport Extreme needs to control NAT.