Which wireless router MAC do I choose for QOS?

Hello, I'm not sure if these kind of questions are suitable here but worth a shot... Wall < Wired Modem/Router < Wireless router So I want to set up QOS on my WIRED modem/router, I want it to limit all data from the wired router, but my QOS settings ask for the wireless routers IP and MAC. But I'm not sure which MAC to use, would it be the LAN, WAN or wireless. My wired modem/router is...