Airport Time Capsule connectivity keeps failing
Starting sometime in early 2015 (I think) I started losing my internet connectivity via my Airport Time Capsule. The WiFi would show that it is connected to my SSID and the unit's light is green, but no internet.
The Airport Time Capsule was connected via CAT5e to my Arris router which came with the cable TV installation, thus creating a two-router / double NAT system in my house which is redundant, but I generally stopped broadcasting my Arris router and just logged all my computers and devices into the Airport's DHCP. For a very long time I never had any issues with this.
When I started losing internet connectivity, it ONLY happened with my iMac, not with my Mac laptop or any of my PC devices or Kindles. If I selected "Turn of WiFi" on my iMac and then turned it back on, it would re-connect with the Airport and then internet gets restored. I started to wonder if my iMac wireless card was starting to go wonky, but it seemed like it wasn't my WLAN that was the problem, but the internet connectivity. I don't understand.
It would lose internet connectivity even when I'm busy using the computer. I would know something went South when all my Mail accounts started showing exclamation points.
Then things got worse. Around early July, possibly coinciding with the 10.10.4 system update, I started losing WiFi connectivity altogether. And in fact now all my computers and networks were struggling to stay connected to the Airport Time Capsule. When I opened Airport Utility and checked it, the Internet would be a yellow light at the top, followed by different arrangements of my Airport Time Capsule and both Airport Express units changing - sometimes with the Time Capsule at the top with the two Expresses beneath (which should be the normal configuration), and then all of them side by side, and so on.... It changed every few minutes, lights turning green to yellow, yellow to green, etc.
So I did a full reset on everything from the ground up. Unplugged Arris router, Time Capsule, etc... Waited... Plugged it all back in starting with the Router. Then rebooted all computers and devices. But it did not fix anything. Behavior continued.
I finally decided to remove the Airport Time Capsule from the equation and logged all my computers and devices into the SSID broadcasted by my Arris router, and suddenly all the problems went away. (Except of course I lost connectivity to my Express modules, and the network drive I had connected to the Airport via USB.)
My question: Do you suspect based on the above information that my Airport Time Capsule's router functionality is going bad?
The drive in the unit still seems OK, though of course it wasn't working via WiFi when the WiFi went down. So I plugged the Time Capsule directly into my iMac via the CAT5 and the iMac immediately found my Time Machine. I was able to make a new backup with Time Machine. And now even that is gone. Airport Utility doesn't see it, and it's not mounted on my iMac anywhere. It doesn't even show up in Disk Utility.
Advice?
I believe my Airport firmware is up to date.
iMac (27-inch Mid 2011), OS X Yosemite (10.10.1)