Thanks for the feedback.
Can you quickly try to disable your Avast and check if spacedesk viewer can discover the server?
By the way, what is your current network profile on your primary machine? Private or Public?
You can check it via Windows Settings -> Network & Internet -> Properties.
Are you connecting to your Home/Office Wifi network? Or is it a public WiFi?
If it’s a home/office wifi network, could you please check if your WiFi router’s setting has network isolation enabled?
This setting prevents direct communication between connected devices, so you have to reconfigure your router and disable it.