As another potential solution that I’ve had running with OpenWRT for many years, the “smcroute” package also works and gives slightly more control over which groups to forward. My configuration in /etc/smcroute.conf is:
phyint br-lan enable mrdisc
phyint br-knx enable mrdisc
phyint br-wlanguest enable mrdisc
mgroup from br-lan group 239.255.255.250
mgroup from br-knx group 239.255.255.250
mgroup from br-wlanguest group 239.255.255.250
mroute from br-lan group 239.255.255.250 to br-knx br-wlanguest
mroute from br-knx group 239.255.255.250 to br-lan br-wlanguest
mroute from br-wlanguest group 239.255.255.250 to br-lan br-knx
to bridge SSDP discovery between three subnets (bridge interfaces on top of VLANs, in this case).
Note: For Mikrotik RouterOS, I had to manually force this port to be marked as a multicast-router on the bridge, otherwise it would not get the IGMP packets with IGMP snooping enabled. I am not sure why, but with “igmp-snooping=yes multicast-router=temporary-query multicast-querier=yes” for the bridge, it works only with “multicast-router=permanent” forced for the relevant tagged-only (trunk) port connected to the Turris. For some reason the IGMP join sent by smcroute on the VLAN(s) doesn’t seem to be recognized by RouterOS to mark the bridge port as a multicast router port.