This config should be even easier :).
No, I just set up VLANs as described in the docs (VLAN - Turris Documentation). I have two TOs in two different locations, one of them is connected to a managed switch only and gets WAN through a VLAN and sends LAN back to the switch on a different VLAN through the same ethernet cable (due to structural reasons). This TO didn‘t have any problems while my other TO had those. IGMP is just an assumption because my main problem was that I was unable to connect to my sonos speakers after roaming between an AP and my TO (and back to the AP, too) and I read that IGMP is used for device discovery in the „sonos protocol“.
If you set up your TO as an AP are you sure you disabled dnsmasq, odhcp and ipv6 router advertisements on this TO?
EDIT: Maybe we should not hijack this thread…