Can you plug in your mcDemo205 with the USB cable but leave off the blue jumper, hit connect gateway while having selected the Testboard Gateway and then plug in the blue jumper to power your mcDemo205?

You should then see it show up in the bottom half of the devices menu after about 5 seconds.

This procedure is also explained in the mcDemo205 User Guide if you have not seen it. https://www.mcthings.com/wp-content/uploads/files/mcDemo205/mcDemo205-User-Guide.pdf