The main router provides the wireless AP (Access Point), that is your wifi network.
The rest are clients.
So the 'router' which is connected to the Arduino should be set as client.
I have them as 'dummy' transparent clients, without DHCP server, without other things. I select "client", let them connect to the SSID of the wifi network, and fill in the password of the wifi network.
You may have to try a few things. In my situation the Arduino itself determines its IP number on the wifi network, and not the "router-as-client". I like that situation, since I can exchange the routers and Arduinos, and every Arduino keeps its own IP number.
However the "router-as-client" does have a settings page and that settings page has also a IP number.
I have a list to keep track of all my local IP numbers, and always select a new unused one. By now that list is 20 numbers long.
I have used this "router as client" with a few brands and with the manufacturers firmware and custom firmware.