The first method, by WiFi is the setup
OK, thanks, got that.
I’ve tried iterations of resets and even using a Thunderbolt connection (adapter) to AE #2 (Which sets up a new network but elimates the pré existing one on the base AE).
If you want to setup the second AirPort to extend using WiFi, then you would not ever use any type of wired connection between the AirPorts to set up the second AirPort. You must use wireless only to set up the second AirPort Extreme. More on that later, once we have the "main" AirPort working correctly.
I think the problem may be in the base AE
This is possible, and the reason why I pointed out that the "main" AirPort must be working correctly before you can extend the network with a second AirPort Extreme.
It’s set in bridge mode coming off the Comcast all-in-one modem gateway.
Bridge Mode is the correct setting for the main AirPort Extreme.
So, in concept am I not just trying to extend the Comcast device vs the AE?
No, not if you are asking about extending the wireless provided by the Comcast modem/router.......unless.......the main AirPort #1 will be located at least a room or two away from the Comcast modem/router.
If the main AirPort Extreme #1 will be located in close proximity to the Comcast modem/router, then you would want to turn off the WiFi function on the Comcast device to reduce the chances of wireless interference that will very likely occur when you have two routers in close proximity both producing the same WiFi signal.
So, my first recommendation......if the main AirPort Extreme will be located in the same room as the Comcast modem/router would be to turn off the WiFi on the Comcast device if at all possible. If that is not possible, post back and we will try to work around this potential issue.
IOW, the create network setting are on the Comcast device, thus AE #2 is trying to extend the network created on the non-Mac device.
Sorry, but I am confused by this statement. We are talking about the "main" AirPort Extreme here....which would be #1. It is #2 AirPort Extreme that will be configured to extend the network that is being produced by AirPort #1.
But first, we need to get your "main" AirPort Extreme.....#1.....working correctly.
Keep #2 powered off for now and we will tackle that setup later.
So, the first question that I have for you is this:
1) Where will be AirPort #1.....your "main" AirPort.....be located in relation to the Comcast modem/router?
The next question is this:
2) If the main AirPort #1 will be located in close proximity to the Comcast modem/router.....are you able to turn off the WiFi on the Comcast modem/router?
Once we have answers to questions #1 and #2, my suggestion will be to start over and set up the main AirPort Extreme #1 again, to make sure that it is operating correctly. Then.....we will set up #2 AirPort Extreme to extend the network using only a wireless connection with the main #1 AirPort.