For what it's worth:
I finally got this to work and I'm not even sure how.
The last sequence I went through was to leave the modem and computer powered up and reset the Airport Extreme box using the reset button. Reset the cable modem when told to (power up/down).
I'm pretty sure this was one of the many reset sequences I went through before.
I also picked a password this time that had no numbers (I know this probably doesn't matter).
One indication that something different happened this time was -
I had the System Preferences panel open to the network panel and noticed that for the first time the IP address and DNS server address shows 10.0.1.2 and 10.0.1.1. Before, it looked more like a Comcast address. Somehow the computer connection was just being passed through to the modem (but incorrectly).