question

Yup - on the bottom facing side of the Cerbo GX. I had a mounted box there - so not easy to see. Connected successfully - a 172.16.x.x address and unfortunately no networked connectivity to any of the existing Internet connections that the Cerbo GX already had. That is a pity.
The written instructions need to be updated.

auto) Locate the QR code sticker on the side of the Cerbo GX
Should read "bottom side"?

manual) Connect to WiFi using the 'WiFi key' which you will find printed on the side of the box …and also on a card in the plastic bag. Keep that in a safe place.
Don't remember getting a card, and it is not on the side of the box - it is on the bottom (or lower) side of the Cerbo GX.

So I guess a modification to make that Wifi into (i) an extension or "bridge" of any Internet network coming in or (ii) if nothing then keep it as is. Be nice to be able to change the name and password too!

I think you are talking about its 'HotSpot' which I suspect is only used for commissioning and emergency access. If so you can find those details at any time by connecting using Victron Connect, bluetooth, and looking at network settings in the app.

Once you have a WIFI connection to the units broadcast SSID you can see the remote console and have the unit join your local WIFI network from there.

As far as I know, and I am no expert, there is no provision for the unit working as a range extender or bridge.

Once you have joined your WIFI LAN, turn off the broadcast SSID, you can always turn it on again with the button on the top if you loose connectivity.

Personally I would recommend connecting with Ethernet, a cable, leaving DHCP enabled and setting a static IP for it in your router. If you cant to that then set the internal IP as static.

However you go about it I am pretty sure that the broadcast SSID isn't what you want long term.