I don't think distance matters all that much. I mean I could be wrong, they're talking about inference, but I have my Zigbee gateway right on top of the router and the Matter Thread stick half a meter away and they're all fine. Connects to devices upstairs and through walls.

I'm sorry, I probably can't help that much, I got a different Matter stick. Definitely can tell you I also spent the better part of a day getting the darn thing to work... The stick was easy but I got a GrapheneOS phone and for some reason a phone with all the Google spyware is needed to link Matter devices... And none of that is obvious, or comes with proper error messages...

But if your network doesn't show up, I guess there's something wrong with the ZBT-2 or the connection between HA and the ZBT-2. As far as I know it comes in Zigbee mode per default, that's why I suggested checking if the firmware is switched to Thread. I think the second step is to go to the Thread integration page, make sure the border router shows up and created some network.

There's a bit of troubleshooting advice in the documentation, but I don't see anything that fits here.