Thanks for the info.
Are you sure there’s no client/AP/wifi isolation on your wifi router settings?
What is the current network profile of your wifi network in your primary machine? Private or Public?
You can check it in Windows Settings -> Network & Internet -> WiFi or Ethernet properties.
If the current profile is Public, please try to switch it to Private.
Can you also try to ping the IP address of your android tablet from your primary machine (and vice versa if you can do it easily)?
To check if both primary machine and secondary devices are reachable to each other.
Please also try to switch spacedesk server OFF-ON (to make sure that it’s running properly).