1.2. Configuring client mode
The first step would be changing the Wrt's behavior from AP to client mode.
nvram set wl0_mode=wet # bridged client
nvram set wl0_mode=sta # routed client
If you are going to leave the wireless interface bridged to the LAN on br0, then you should choose wet. If you are going to configure Wrt as a router, so that the wireless interface is a pure client and the LAN is on a completely different subnet, then choose sta.
NOTE: As soon as your AP is in client mode you can't connect any wireless clients to it anymore because it's not in AP mode (wl0_mode=ap). Also note that wl0 above is WL in lower case followed by Zero.
There are two different client modes: bridged and routed. They are mutually exclusive.
1.2.1. Bridged client mode
In bridged client mode, all computers connected to the client will be connected to the subnet of the access point you are connecting to (no firewalling, unless you use ebtables).
When using the bridged client mode, you should disable the DNS/DHCP server: If the "chmod" command fails and reports a read-only file system you will have to remove it instead (you can always put it back since it's just a symlink to the real script).
chmod -x /etc/init.d/S50dnsmasq
to put back to the way it was do
chmod =rwx /etc/init.d/S50dnsmasq
If your configuration was previously set to "routed" client mode, you'll need to add the wireless interface to the bridge again and remove it from the wan interface.
IMPORTANT: Use the correct network interface names for your hardware version.
nvram set lan_ifnames="vlan0 eth1"
nvram set wan_ifname=vlan1