Make wifi gateway ip the lan dhcp client ip

Hi guys
I'm a newbie to OpenWRT, still learning how to configure and use it... I am facing a problem with configuring my WiFi interace on Raspberry pi 3 b+. I use the latest release OpenWrt 18.06.5, so my problem is that i want to configure my wifi interface to use as a gatewy the ip that the lan interface get when get connected to the main router and to have it's own ip range. i know this is logic but i suck at networking and i need your help.
please guide me through this and thank for anyhelp

This is a very confusing description. Maybe a diagram would work better?

Thank you for your quick response and sorry for late reply
this is a way that i hope i can explain to you what i try to do

I think you want to configure the pi as a dumb ap.

Do not try to use the op range 137.164.1.x because that is not a private IP range and could mess things up.

Hi mate
From the first look it is what i'm looking for but i want the pi to be able to control clients based on MAC.
to entirely describe what i try to do : i want to create a captival portal with the pi using nodogsplash so every clients can login with their username and password for a specific amount of time with mac adress control because the ip associated to each client can change based on connect/disconnect from the network so is the answer you provide for dump AP can help for what i try to do thank you so much for your help.

I am not familiar with any of the methods on openwrt to achieve what you are trying to do. I know that the unifi aps have the ability to do much of this stuff, but that uses their own firmware and associated controller.

Hopefully someone else will be able to give specific advice.