Could you please try to connect via HTML5 Viewer on your iPad?
Just open a web browser (e.g. Safari or Google Chrome) then go to http://viewer.spacedesk.net/, then try to connect to your primary machine’s IP address.
Thanks for the feedback.
It seems like this is a network discovery issue on the iOS viewer app.
Could you please check on your iPad Settings -> Privacy -> Local Network, if spacedesk is on the list and enabled?