Hi,
I want to use my OpenWrt to relay / extend my network, both thru LAN ports (I dont need WAN) and Wifi.
My main network is 192.168.0.0/24, I want to plug the router either by WAN or LAN port, so he could create a Wifi network relaying DHCP to my DHCP server. Wifi address should be in same network.
relay is usually for passing dhcp from one subnet to another.
if you want the same network ( easier )... then just disable dhcp on it... give it a static ip ( on your lan range ) so you can get to it.... and plug into upstream via a LAN port...
A wired connection is straightforward, the configuration is called a dumb AP. Basically there is one network inside a dumb AP, and it has direct access to all the resources offered by the main router without doing any routing or firewalling of its own. This is called a bridge not a relay.