Is the guest WiFi created by your openwrt router? And is it working with a wired connection currently? If so, what is the purpose of adding a WiFi uplink?
This should be easy. The connection should not be bridged with the wan. It should be routed. The firewall is used to allow the routing to the wan but prohibit connections between the lan and the guest network. Have you followed the guest WiFi tutorial?
Feel free to post your configuration for review and assistance:
Please copy the output of the following commands and post it here using the "Preformatted text </> " button:
Remember to redact passwords, MAC addresses and any public IP addresses you may have:
Actually one thing that is not clear is if you are using multiple routers, and if the openwrt one in question is operating as a dumb ap. If this is the case, the process is a bit different.