Access Sonos/SmartTV from different network

I have a Flint2 router with Openwrt 21.2 and created 3 wireless networks

  • lan - Trusted devices
  • iot_online - IoT devices with access to internet
  • iot_offline - IoT with no access to internet

I’ve placed the smart TV and the Sonos speakers in the “iot_online” network and laptop/phones are on the “lan” network.

Network configuration

config device
	option name 'br-lan'
	option type 'bridge'
	list ports 'lan1'
	list ports 'lan2'
	list ports 'lan3'
	list ports 'lan4'
	list ports 'lan5'
	option igmp_snooping '0'

config interface 'lan'
	option device 'br-lan'
	option proto 'static'
	option ipaddr '192.168.8.1'
	option netmask '255.255.255.0'
	option ip6assign '60'
	option isolate '0'

config interface 'iot_offline'
	option proto 'static'
	option ipaddr '192.168.20.1'
	option netmask '255.255.255.0'
	option device 'br-iot-offline'

config interface 'iot_online'
	option proto 'static'
	option netmask '255.255.255.0'
	option ipaddr '192.168.10.1'
	option device 'br-iot-online'

config device
	option bridge_empty '1'
	option type 'bridge'
	option name 'br-iot-online'
	option igmp_snooping '1'

config device
	option bridge_empty '1'
	option type 'bridge'
	option name 'br-iot-offline'

Relevant firewall rules

config defaults
	option input 'ACCEPT'
	option output 'ACCEPT'
	option forward 'REJECT'
	option synflood_protect '1'

config zone
	option name 'lan'
	option input 'ACCEPT'
	option output 'ACCEPT'
	option forward 'ACCEPT'
	list network 'lan'

config zone
	option name 'iot_offline'
	option forward 'REJECT'
	list network 'iot_offline'
	option input 'ACCEPT'
	option output 'ACCEPT'

config zone
	option name 'iot_online'
	option forward 'REJECT'
	list network 'iot_online'
	option output 'ACCEPT'
	option input 'ACCEPT'

config forwarding
	option dest 'wan'
	option src 'iot_online'

config rule
	option dest_port '67-68'
	option src 'iot_online'
	option name 'Allow_DHCP_iot_online'
	option target 'ACCEPT'
	list proto 'udp'

config rule
	option dest_port '53'
	option src 'iot_online'
	option target 'ACCEPT'
	option name 'Allow_DNS_iot_online'

config rule
	option dest_port '67-68'
	option src 'iot_offline'
	option name 'Allow_DHCP_iot_offline'
	option target 'ACCEPT'
	list proto 'udp'

config forwarding
	option dest 'iot_offline'
	option src 'lan'

config forwarding
	option dest 'iot_online'
	option src 'lan'

My phone with the Sonos app, or accessing the TV, doesn’t seem to work anymore. In the Sonos App I cannot discover the speakers in “iot_online” anymore. Also streaming an app to the TV doesn’t seem to work anymore.
I suspect it may have to do with IGMP not being forwarded as expected but I’m not familiar enough with it.
Does anyone know if it is possible to get this work?

I would assume that the problem might be the mDNS detection.