I was directed here from this article (https://techwiser.com/use-your-android-as-second-monitor). There they say that this tool can be used with USB. Is the only way to use this with USB to do USB tethering? Or can this be done only through the USB? Not using internet to connect the two devices?
If your secondary machine is android, you can also try the android debug bridge cable connection.
For details, please check the Performance Tuning -> Network Bandwidth -> Android Debug Bridge Cable chapter in the user manual https://www.spacedesk.net/user-manual/.