My Denon receiver is not visible on our iOS devices.
After a cold restart of the receiver (power off, unplug, wait, plug), it works during few minutes and stops working.
The culprit was "IGMP Snooping Status" on my Netgear Switch (JGS524Ev2).
Just after disabling it, the receiver appears.
Connect to your switch, go to System > Multicast > IGMP Snooping
Set "IGMP Snooping Status" to disable and Save the configuration
It should also work for the other Netgear ProSAFE (JGS516v2)
I've read a lot of threads that advise to enable IGMP Snooping on the switch to make Airplay work.
So, i set it back to enable and i also enable "Validate IGMPv3 IP header".
I looked at my Ubiquiti Unifi Wifi access point and i found that "Enable multicast enhancement (IGMPv3)" was not enabled.
After enabling it, it seems to work with IGMP Snooping enabled.
On Ubiquiti Unifi console, you can modify "Enable multicast enhancement (IGMPv3)" option in global settings > Wireless Network > Edit your Wifi_Network > Advanced Options