I got UI3 working, and am reporting it here in case this helps somebody else who finds their way here.
For some time I could not browse to the IP address of my BI PC, with the port appended to the IP, as called for by the BI manual.
Seemed like this should just work, but wouldn't, and this alone should have told me this is a LAN issue.
Turns out the PC was not ready to be found on my network, because prior to installing BI, I had done a clean install of Windows and everything was at defaults.
Once I connected the dots, a couple of minutes is all it took.
Now I can browse to BI on my LAN, including from mobile devices.
I still have to test this from outside my LAN, but now with zerotier I should be able to safely connect to BI from outside my LAN.
Note to self: Get BI access operational from other devices within LAN, prior to trying remote access.