There are two ways bridging might work here.
Both require to access the device via ssh.
- put the modem interface into the LAN bridge:
uci set network.wan.type=bridge
uci add_list network.wan.ifname=m0
uci commit network
You can also use "vim /etc/config/network", to make the changes in an editor.
Now some port interfaces have be added, that would require to single out interfaces of the switch (changing the switch config in /etc/config/network). You can also put the entire switch interface into the wan bridge.
But the access to the router via ssh might be difficult (open firewall for wan in /etc/config/firewall).
The lan bridge might not be the best fit here instead of the wan bridge, because that is where the dhcp server on the router runs on.
- use a pseudo bridge, in case the the modem interface does not support bridging:
https://wiki.openwrt.org/doc/recipes/relayclient