I am wondering if there is any luci-app or passwall plugin that could show the public IP addresses of the interfaces (such as WAN,WWAN,TUN0,WG0,....).
There is an alternative to official openwrt (called kiddin) which its bundled passwal does come with a plugin or small embedded code that shows the country the connected proxy of the passwall which is very handy.
I am wondering how I can add that feature to passwall or if there exists any better alternative as I have mentioned that can show the flag and public IP addresses for different interfaces?
Assuming the IP on your interfaces are actually Public, the LuCI web GUI already does this in default OpenWrt.
Connected proxy?
To be more clear, are you actually seeking a software that performs some kind of "what is my IP" lookup via each interface to find the Public IP and a.) country of the Autonomous System or b.) the country of ISP allocated the Public addresses?