The setup wizard. It’s accessible from the web interface.
Wow, lots of interchange on this since I asked the question just four days ago! And thanks to that interchange between you and gregsapphire, I've gotten to the wizard!
It turns out that my installer set up cellular for the Backup Gateway Internet connection. I saw the TEG-xxx in my WiFi list after seeing someone else mention it in another forum/thread. I was able to connect to that network and get into the Powerwall UI using the IP address of 192.168.91.1, which is apparently the default IP address for the cellular connection based on what I've seen others mention. I was able to log in using the serial number of the Backup Gateway, although that was a little confusing because my installer also appended an "S" to the front of the serial number. Also, one of the digits/characters is actually an upper case "I" (eye), not a "1" (one) or a lower-case "l" (ell). So my password is ST17I0008xxx, where xxx is the last 3 digits of my Backup Gateway serial number.
After logging in to the UI I was able to run the wizard. I noticed the warning that running the wizard will shut down the Powerwall and that apparently the only way to start it up again was through the UI, so I was really hoping that this was going to work! I went out to the garage and saw that the green lights on the two Powerwalls were both flashing, so that's apparently what they do when they're shut down. Screen 1/11 allowed me to change the Internet connection to WiFi, so I selected that. Just for the heck of it I also went through all of the screens and changed the name of my installation to a more personalized one. Then I reconnected to my local WiFi network, looked in my router's DHCP Client Table and found the IP address that the Powerwall was using. For good measure, I also went into the router DHCP Reservation Table and reserved that IP address and gave it a more friendly name (Tesla-PW2 instead of 1118431-00-G--T17I0008xxx).
Then I browsed to the IP address of the Powerwall on my local network and was again at the Powerwall UI! Success! I started the Powerwall, went to the garage and heard some clicking and then the Powerwalls started humming and went into their glow sequence, so they came up again with no problem.
The reason that I wanted to access the wizard and get it on my local network was to be able to set up data aggragation using PVOutput.org as documented on the MikesGear.com site:
Monitoring Tesla’s Powerwall2 on PVOutput.org. So my next project is to get that up and running. It needs an always-on PC however, and I saw somewhere that someone set his up using a Raspberry Pi, so that's another task to get it up and running on that.
While I was mucking around in my router I noticed that the firmware was outdated so I took the opportunity to upgrade it. After the router rebooted I discovered that the Backup Gateway had reverted back to the cellular connection, so I had to go back in and change it back to WiFi. We have a long trip coming up and that got me to wondering what would happen with the Powerwall Internet connection if my network went down while I was gone. Apparently, Tesla has that covered.
Something interesting that I noticed was that when I looked in my router's outgoing traffic log, the Powerwall IP connected to 69.89.207.99, which I assumed was Tesla. Imagine my surprise when i got a standard Apache2 Ubuntu Default Page! Not sure what my Powerwall was doing at that page and how Tesla is getting the data, but it makes me wonder how secure their data collection setup is if they didn't even bother to change the default page.
Many thanks for your help and the interchange between you and gregsapphire!