To further analyze the discovery issue, could you please run the spacedesk Driver Console app on your primary machine, then click Diagnostics -> Save All Information button, then send us the diagnostic logs.
Just right-click the folder -> Send to -> Compressed (zipped) folder, then attach the *.zip file on your next reply.
As for the native USB cable connection, spacedesk does not support it.
But if your Android tablet supports USB tethering, you can connect your spacedesk Viewer to you primary machine over USB tethering connection.
For assistance, please check Performance tuning -> Network Bandwidth -> USB tethering chapter in the user manual https://www.spacedesk.net/user-manual/.