Luci shows IPv4 Upstream interface as connected, but the relevant link is down

please check this Luci screenshot that shows IPv4 Upstream interface br-wanb as connected.

However, this interface is physically not connected, there's no link.
And this is confirmed by CLI.

Can you please explain why Luci display is inconsistent to reality?

LuCI acquires its information from netifd (ifstatus wanb / ubus call network.interface dump) and in this case it appears that netifd is considering wanb to be logically up/connected. It might help to not let wanb be a bridge, this will remove an unneeded layer of indirection and lets netifd work with the link state directly.

Also you're using proto static, this defaults to ignoring carrier events (it is supposed to be static after all). To let static proto interfaces logical connected state follow the underlying link state, you can unset the "Force Link" option:

On the command line / uci it would be

uci set network.wanb.force_link='0'
uci commit network
ifup wanb