This mode is NOT for WIRED connections between two routers! It is a wireless connection between two routers only.
A router in Client Mode connects to another wireless Access Point (the host router). It uses its wireless connection as the WAN interface, and shares the internet connection only to the LAN ports. It is not seen as an access point by laptops or other computers scanning for AP's and does not accept wireless connections from client devices. The AP is not required to be running DD-WRT firmware.
The main router and secondary router (DD-WRT client mode) are on separate subnets. NAT is used between the routers. Thus, when port forwarding is needed it must be configured at both routers — not just on the main (host) router.
The router in Client Mode must use its own DHCP server to give out IP numbers, Gateway and DNS server to the devices connected to its LAN ports.
To have computers connected to both routers (main and secondary) and co-exist in the same subnet, set up DD-WRT as a Client Bridge, Repeater Bridge or use WDS. Further explanation of bridging modes is in the Glossary.
Client Mode Setup
The Router/AP that you are trying to connect to is the primary router. The Router you are configuring is the client router. You don't need to worry about the primary router as long as you set your client LAN IP address to a different subnet. What this means is, if the primary router has an IP of 192.168.A.x, you need to set the client router to an IP of 192.168.B.x. Most primary routers will be at 192.168.1.1, or 192.168.0.1. So, when you are configuring your client router, set it to 192.168.2.x.
With that in mind, here is how to configure it:
Download the current recommended build for your router. Check the forum for your chipset to find recommended builds and general information related to your hardware. If you are using a Broadcom based router, read the peacock thread prior to starting. DO NOT USE SP1!
Do a HARD reset on your router.
Connect a cable from your computer to the LAN port on your router.
Set your computer to a static IP address (e.g. 192.168.1.7)
Open a web browser and connect to 192.168.1.1 to view the DD-WRT GUI.
You should be asked to change your password and username. Carefully type these in. If you aren't asked for a password and username, do a hard reset, this time doing it properly! Hit change password.
FIRST go to Wireless -> Wireless Security and enter the Security Mode and other key information that matches your primary router that you will be connecting to over wireless.
Hit SAVE (not apply)
NEXT go to the Wireless -> Basic Settings and change the Wireless Mode to Client.
On that same tab set the Wireless Network Name (SSID) to exactly the same SSID as your primary router you will be connecting to over wireless. Make sure spelling and capitalization match. Note: If there are multiple routers broadcasting the same SSID, and you would like to connect to a specific one (IE, the primary router, instead of the repeater), enable MAC Filtering on the Wireless Tab, and add the MAC Address of the correct Access Point. The MAC Address can be found on the "Site Survey" page, linked from the "Wireless" Sub-tab of the "Status" Tab. - Conjur
(Optional) Set the correct ack timing in meters. (Where?)
Hit SAVE (not apply)
NEXT go to Setup -> Basic Setup and change Connection Type to Automatic Configuration - DHCP (most likely, but see NOTE below).
On that same tab in the Network Setup section set the Local IP Address to 192.168.2.1 (Your LAN subnet MUST differ from the WAN subnet!).
(Optional) Set Static DNS servers in the Network Address Server Settings section if your WAN type does not provide them or you want to use different ones than the AP provides.
(Optional) Check Assign Wan Port to Switch if you want to be able to use it as another LAN port.
(Recommended) Change your Time Zone and DST to match where you are.
(Optional) Set a Server IP/Name for in Time Setting section for NTP (it will use a hidden default server if you leave it blank).
Hit SAVE (not apply)
(Optional) Go to Security -> Firewall and disable the SPI Firewall if you do not need to worry about security between the AP and Client router.
Set your computer back to auto IP and auto DNS if you like. You will get a DHCP address from the router.
You are done and the Client router should connect to the AP within a minute or so. You may need to unplug the router for 17 seconds if the router doesn't reset. After you ensure that it does connect and work properly then you may configure any other settings you wish to (Virtual Interface for wireless connection for example).
NOTE: If the client router is externally managed, such as in a college dorm etc., you should Go to Setup -> Basic Setup and set the WAN port protocol to what you need for the AP (mostly static, dhcp or pppoe). This is the setting that is bonded to the Wifi interface. Fill in the needed info provided by your ISP if this AP is externally managed. Note: This is what a computer (wired or wireless) connected to the ROUTER would use to gain access to the network, not to be confused with the settings required to gain access to the WAN by the ROUTER. Also note that the device should be in GATEWAY MODE instead of ROUTER mode, otherwise Masquerade/NAT does not happen.