There’s an occasional bad interaction between Android, networking hardware, and Roon’s use of IP multicast to discover remotes and endpoints. Some people have been able to fix it permanently by changing IGMP settings on networking gear, in my case by turning off IGMP snooping on a Netgear managed switch. There’s also a temporary fix: when the Android device fails to connect, the error screen has an option to enter a different core’s network address. Enter the broadcast IP address 255.255.255.255 rather than your core’s name or address there, and that may work, at least until the Roon Android app starts again.
I encountered this Android - ROCK issue yesterday when changing my router from an Airport Extreme to a Synology RT2600ac.
When new router IP allocation was set and Roon folders pointed to the changed NAS IP paths, 3 android devices in the house could no longer find ROCK.
1 phone found ROCK after around 10 minutes.
1 tablet found ROCK after deleting and reinstalling Roon Controller app.
My wifes phone refused to find ROCK at all.
I then turned off IGMP Snooping in the Synology router and her phone found ROCK within 30 seconds.
IGMP snooping is still turned off - I admit I have not researched what IGMP is / what it does but with it off everything on the network is working so it can stay off.
Roon devices use IP multicast to advertise to the rest of the local network “I’m here, let’s connect!” IGMP is a network management protocol intended (among other things) to adjust multicast traffic when the same multicast routes are used repeatedly. IGMP snooping is when a router or server looks at the surrounding multicast traffic, makes a note of it, and uses that information to restrict presumed excessive traffic. This all makes sense for large networks, but is really irrelevant for a small home network, and seems to interfere with Roon’s use of multicast discovery for some devices. There’s no risk in turning off IGMP snooping on your home network.