How to make WAN-to-LAN on OpenWRT so that the router has its own IP?

Question

Network routing

The situation is standard, but not very common :)
Before that, I used a router with OpenWRT simply: The input cable from the main router (MikroTik) is in the LAN, disabled DHCP in OpenWRT, everything works.
But I bought another router (Mi Router 4C) and noticed that it has a "home network extension" mode in its settings. That is, he himself receives IP, and allows devices to receive IP from the main router via WiFI \ cable.
Inside it, I also suspect OpenWRT.
Therefore, the question is: how to do the same on OpenWRT? Because now the router does not receive an IP, but simply works like a switch.
How did Xiaomi make it so that the router broadcasts traffic to the main router, but at the same time has its own IP?
(Answers Preferably with screenshots)