Could you please check the following:
– both primary and secondary machine are connected to the same local area network
– spacedesk server is ON (you can check via spacedesk Driver Console)
– Is there a VPN(Virtual Private Network) currently active?
– Do you have third party firewall or antivirus software?
For further details, please check the Troubleshooting -> Network Connection chapter in the user manual https://forum.spacedesk.netuser-manual/.