It looks you are using the router as a switch and AP, and the multicast traffic floods your wireless interface.
Multicast is different from broadcast, as it is only intended for subscribing hosts. Unfortunately, any switch that doesn't implement IGMP snooping doesn't know who wants multicast, so will handle it like broadcast.
The ideal solution is to implement IGMP snooping. I don't know how to do that.
A working solution is to filter multicast traffic being sent to the wifi interface.
1 - Install ebtables:
opkg update
opkg install ebtables
note 1: if you are using the router as a switch, you will need to add config option gateway and config list dns to the LAN interface configuration to gain internet connectivity.
note 2: installing kernel modules with packages doesn't work every time. If your router doesn't boot anymore, please compile OpenWRT with ebtables built-in.
2 - Add rules:
Edit /etc/firewall.user and add:
ebtables -A FORWARD -o wlan0 -d Multicast -j DROP # drop multicast traffic coming from another interface to wlan0(WIFI)
If this doesn't work as you wish, you may try also this rules:
ebtables -A FORWARD -i eth0 -d Multicast -j DROP # drop multicast traffic coming from eth0(LAN) to any other bridged interface
ebtables -A INPUT -i eth0 -d Multicast -j DROP # drop multicast traffic coming from eth0(LAN) to the CPU
Regards