The most common cause of fluctuating lights on the router is no internet connection. If the connection is good and still you find Apple Wi-Fi router blinking yellow, it’s because there is a firmware update to be installed. You can try resetting the device by pressing the reset button on the router. You will have to set up the router again because it will be back to default factory settings. You can also go to the settings, check for firmware update and install them. Reboot the router and see if the yellow light has stopped flickering.