I am experiencing the same problem as him.
I am trying to extend the screen on my surface laptop and fire hd 8.
I have succeeded with wireless but failed with wired connection.
I will describe what I have tried to improve the situation.
The USB surface laptop has USB type-A 2.0 and mini displayport and the fire hd 8 has USB type-c.
First, I tried to connect both devices USB to USB.
Both devices made a connection sound, but the screen did not extend.
The USB connection was not recognized on spacedesk either.
Next, we tried connecting via mini displayport.
When connecting the cable, we used a capture board so that the direction of the output was in one direction, but the screen was not output.
However, other applications were able to make a wired connection this way.
In addition, we restarted and reinstalled the app, unplugged and plugged in the cable, and restarted the device, but could not do so.
I would also like to use it without wi-fi.
Thank you in advance for your help.