iPhone App cant find the core

Hi there,
I have put Roon Core on my NAS (DS418play), which is wired, and my mbp2015 could connect to the care normally via wifi without any specific configuaration. Yet when I tried to use the iOS Roon remote app, it kept looking for the core. I have deleted and reinstalled the App, but it did not work at all.
Does anyone have any suggestion?

The obvious thing to look for, is making sure you are connected to the same WLAN as your MacBook Pro. Most likely you re on 4G or similar…

…or a restart of core machine and/or remote device (iPhone).