Is your ISPmodem connected with the WAN port of your OpenWRT router ?
I've blocked access to my bridged modem on my main router for the guest/public network, so i guess it should work for you as well.
If you truly want to isolate your public network clients i would suggest to take a look at my rollercoaster Thread about wifi+wired client isolation (in my case over two Routers)...
I've started with OpenWRT + Tomato and ended up with OpenWRT + OpenWRT.
At the end it finally worked out: How to prevent Guest Network clients to communicate with each other? - #124 by Kherby