Flashing OpenWrt through the OEM web interface is not always possible but we can use TFTP instead.

Some months ago I configured a OpenWrt WiFi router behind a Fritz!Box. A friend of mine told me that he is able to connect to other network clients outside of the OpenWrt guest WiFi.

The behavior is quite clear after a minute of reflection. The Fritz!Box is connected to the WAN port of the OpenWrt router ;)