Airport Express and Time Capsule Not Playing Nice
The Airport Express is connected to the modem by ethernet cable and the Airport Express is set to create a wireless network. The Time Capsule is set to extend the wireless network. When I am connected to the AE, I have no issue browsing the internet, and I can ping both the AE and TC. When I connected to the TC, I cannot ping the AE, cannot browse the internet, cannot ping google, cannot ping google's DNS, etc. The only thing I can ping when I am connected to the TC is the TC, and it behaves as if it is not connected to the internet at all. Even though if I connect back to the AE, I can ping both devices no problem.
Airport Express 7.6.4
Time Capsule 7.6.4
No history of connectivity problems before upgrading the firmware on both devices to 7.6.4. I'm sure the common advice will be to just roll back the hardware to 7.6.1 or some such, but I've tried that and the problem persists. I've searched all over trying to figure out if there is a setting that can cause this problem, but no luck finding anything helpful so far.
I've also tried factor resets, reinstalling the 7.6.4 firmware, rolling back the firmware, and tinkering with nearly every setting using the old 5.6 airport utility. Nothing seems to work.
Time Capsule-OTHER, OS X Mountain Lion (10.8.5)