How to configure DD-WRT Router as Access Point
You maybe have equipped your older router with DD-WRT and just want to use it as an access point? This configuration is quite simple.
First you go to Wireless > Basic Settings and chose AP in "Wireless Mode".
Then we change to Setup > Advanced Routing and pick the option Router instead of Gateway.
We then go into the "Basic Setup" tab and set WAN Connection Type to "Disabled". Because your main router does connect to the WAN or Internet and this second one only operates in the internal network. Now can "Assign the Port to Switch", so get a fifth port.
What you have to do now is to enter the local address of your main router as Gateway and DNS, so that everything that has to go out into the internet finds its way. In this screenshot these addresses are 192.168.3.1, because this access point runs in this specific subnet, but it can very well be that for you the standard 192.168.1.1 (and therefore 192.168.1.2, 192.168.1.3 etc.) is correct.
"Local IP address" now is the address of this access point. So if your main router is .1 at the end, this may be .2 (if the address is available). Subnet mask as in the picture.
Also important: connecting devices should get an IP address automatically. So keep it simple, we just use the DHCP server from your main router that gives out these addresses. So chose "DHCP forwarder" and the address of the main router (normally same as Gateway and DNS address).