So...I added the full IP address, tested it, and BADDA BING, BADDA BOOM, I now have a secure HTTPS connection to my BI box from anywhere in the world."If you receive an error remotely stating “successfully tunneled to your NGROK client, but the client failed to establish a connection to the local address localhost:81” it may be that “localhost” is not resolvable for some reason. In this case you may use the full LAN IP address in the NGROK command. Edit the BAT file and change the 81 to your full LAN address such as 192.168.0.200:81 for example."
Thanks again to the BI support team for this. I beleive this is also being added to the manual.
And as a demonstration, please see the image below. You can access it from anywhere via http or https, internally or externally.