It isn´t possible to use the IPR/S 2.1 in an IGMPv3 network because the router doesn´t support the protocol version 3.
Additionally KNXnet/IP doesn´t guarantee permanent data traffic which results in exceeding timeouts. This affects IGMPv1 and IGMPv2.
The IPR/S 3.1.1 only supports IGMPv3. Timeouts aren´t relevant in IGMPv3 because the switch polls.
Further information about IGMP: https://en.wikipedia.org/wiki/Internet_Group_Management_ProtocolTags: Interface, IP