Hi Nigel, IGMP snooping is a method where by devices listen or snoop on the multicast protocol being sent over specific switch ports to ascertain whether devices connected on a particular switch port need to receive specific addresses or groups of multicast data. If the devices on the port are not subscribing to particular multicast address group then the data is pruned from that port. This is for network loading and efficiency reasons and is usually quite important for wifi, since multicast data is otherwise sent to all clients on a network like broadcast data.
UPnP Simple Servive Discovery Protocol uses multicast address of 188.8.131.52, and for discovery to work that address needs to be received by all devices on the network. Now devices participating in the discovery will join that address group or multicast group by using SSDP IGMP discovery messages. A dumb basic switch like a little Netgear ignores this and sends the discovery data to clients whether they want it or not. Devices operating IGMP snooping will only send the discovery data to devices that have requested to to be part of Simple Services Discovery Protocol group ( i.e. Participating with the 184.108.40.206 multicast address). Hopefully you can now see why if something goes wrong with a devices IGMP group request and the snooper that a device will not become discoverable even though it's unicast IP address works fine on the network. The Naim devices and the UPnP media server will be effectively issuing and responding with discovery group messages .
An IGMP querier is simply a method of actively forcing devices to respond if they are part of a multicast group so the data can be more accurately snooped, and so multicast group addressed data like SSDP becomes more responsive .. in other words my Naim app becomes very responsive to all valid devices on the network.
With regard to the advice given to you.. I don't understand it as stated as you definitely don't want to add a router as you suggest.. the network won't work, but you might want to add a wifi access point like a little Apple Airport Express off the 2960 switch, or perhaps you can get your old non BT broadband router simply to act as a wifi access point... perhaps referred to as a wifi bridge?