Step 1: Different Network Segments

If Primary Machine and Secondary Machine are in different network segments, then they cannot discover each other. 
However, they may still be able to connect. In that case try to add the IP address to detect the primary machine by clicking the ” + ” button.

To get the IP address of the Primary machine please check the above chapter Network Discovery .

Step 2: third party firewall software

In case the above steps did not help, then a third party firewall software (other than Windows Firewall) could still prevent detection. Please check this chapter’s paragraph ” Firewall Settings ” below.

Step 3: WiFi router’s network isolation active

In case all above steps still did not help, and if this is happening on a wireless network, then there might be one other remaining problem: WiFi router’s network isolation feature might be activated. The only solution for this is re-configuring the router.

Step 4: VPN (Virtual Private Network) enabled

Just refer to the VPN topic below.

Step 5: Network profile

In some cases, setting the network profile from “Public” to “Private” solves the spacedesk network discovery issue.
To set, open the Windows Settings -> Network & Internet -> properties of the current network, then select Private underneath Network Profile.

Step 6: Alternative ways to connect

If all above connection attempts failed. 
Try USB Tethering or Wifi Direct / Hotspot setup to establish a network connection between Primary and Secondary machines.