I unchecked the box “Disable IGMP Proxying”.
Disable IGMP Proxying
The IGMP Proxying function lets a LAN PC to receive the multicast traffic it is interested in from
the Internet. You can click this check box to disable the function if you do not need it.
I believe the intended effect was to enable the IGMP proxy function from the internet to the LAN side. I have no idea how this affected muticast traffic from one host to another on the LAN side but my laptop is now able to consistently find the roon server. The Netgear description does not explain that the parameter impacts LAN to WLAN traffic. You can close the issue although I don’t understand how this worked.