I've tried turning off firewall already (completely). But I just tried what you said and disabled it only for that WIFI network and it's not working. Any other ideas?

Can you show your LAN and zt-ip range? Personally, I think it is the reason of the ip subnet mask. The subnet judgment of windows is different from that of linux. I have encountered a similar situation before, and finally solved it by modifying the subnet mask of windows.

Did you manually add a route between the two subnets on openwrt?