There might be a simpler fix than all the above gyrations/downtime. There was for me.
I now have an Arris TM722G/CT. I like that it doesn't have wireless nor functions as a router. Simple data/telephony only. My previous router had those features, but had to be configured in Bridged Mode to work.
After a major period of intermittent and/or uninterrupted downtime (10 days), the service outage in my area was fixed. I'll withhold the unpleasant details.
Tonight, after messing with various cable connections between laptops, Cisco SG 200 switch, and Airport Extreme, I found a reasonably quick and effective solution.
Note 1: Before reconfiguration, I shutdown my Airport Extreme (APE) and relocated it next to my Mac, so it was powered down for about 10min.
01) Connect your laptop or PC directly into the WAN port on the Arris device;
02) Follow the steps Ron provided above in System Preferences --> Network
03) This should dynamically assign the IP address for you to test the PC Internet connection;
Note 2: Your local IPv4 address and router address will likely be different for the Airport Extreme (more below)
04) Swap out the direct cable connection from your Arris device from your laptop to the Airport Extreme (APE);
05) Run a new cable (or try wireless) from the LAN port on the APE to your laptop;
06) Turn on the APE. The light should turn a steady Green since it's connected to your PC (LAN);
07) Launch Airport Utility;
08) From Internet tab, select DHCP from Connect Using: drop down;
09) From Network tab, select DHCP and NAT from Router Mode: drop down; (This assumes the same device as me; otherwise, Bridged Mode);
10) Click the Update button and wait for APE to reboot;
11) Both APE device and Internet lights should show Green;
12) Test network connection on connected laptop - should work. It did for me.
Note 3: From my PC (Note 2 above), my local IPv4 router address was 73.x.x.1 with subnet of 255.255.254.0. When reconfigured for use with the APE, the IPv4 router address on the APE was a 98.x.x.1 address with subnet of 255.255.252.0. My understanding of networks is inadequate to explain why this happens, but I suspect Comcast knows whether the modem is connected to a PC or a router. LAN side IP addresses of devices connected to the APE were consistent with previous configuration as I did not change the IP range for the LAN.
Hope this is a viable option for those of you with the same issues as others following this thread.