Block clients from seeing clients from wan. Luci

I have one modem and one router. The modem is connected to the router as wan. I want to block all those clients from my router to see clients from my modem. My modem has limited functionality so I have to configure from my router.
I don't know much about firewall...
I prefer to configure the router from Luci interface, thanks.

I assume you have a double NAT setup and your OpenWrt device does have a private ip on it's wan interface, right?

One question before you get any answer...
Why you wanna have some devices connected to your "modem" instead of only your OpenWrt device and connect everything to this?

Quite simple. The modem is in my house, the router is in my garage.