I have a Samsung Galaxy Tab A 10.1" that I want to use as a second monitor with my PC. Spacedesk works great for my needs so long as I'm on a network with lots of bandwidth -- but more frequently I want to use second-monitor functionality in other environments, e.g. at a coffee shop, where the number of users bogs down network traffic and makes Spacedesk unacceptably slow.
Spacedesk doesn't natively support a wired connection, but for those who want one they suggest using USB tethering, presumably so both devices are on the same network. Problem is, my tablet is the Wifi-only version, so USB tethering isn't enabled. I rooted the device and used adb to run the commands enabling USB tethering as an option. It didn't take - the option never appeared under Settings / Connections. Then I installed ClockworkMod Tether as a 3rd party usb tether app. The PC tether software said it connected successfully to the tablet, but Spacedesk couldn't connect. Next I tried "gnirehtet", a reverse-tethering app. Got it working fine, but again Spacedesk on the tablet couldn't find the Spacedesk server on the PC.
How can I get both devices to be on the same network, without an intermediate connection (e.g. a router), in a way that preserves my PC's ability to connect to wifi, and that will allow Spacedesk to connect?
Turns out there's this thing called "Mobile Hotspot". Of course I couldn't use this on my tablet, as it had been disabled (thanks Samsung). But you can set it up on Windows easily -- in Windows 10 it's under Settings / Network & Internet / Mobile Hotspot. Connect your tablet to that hotspot and Spacedesk works like a charm.
Huge thanks to SpaceDesk customer support, who were extremely helpful in troubleshooting this apparently rather unusual situation. 5 stars both for the product, which I can now use anywhere, and the customer service.