Often presenting as an NSURLErrorDomain -1009 message, this is caused by “Network discoverability” being turned off for the App being used.
This functionality is required for any App that accesses any tcp/udp port on your local LAN. I have come accross it in owntracks, homeassistant, and immich before.
To rectify in iOS 18 navigate to Settings > Privacy & Security > Local Network then enable the checkbox next to the specific App that you are receiving the error in.