This light indicates whether the device is receiving power and functioning correctly. What Can I Do? First, please check that the optical cable which comes. Red optical light on the ONT means there's no light signal from the fiber. You'll need a tech out to get it fixed, unfortunately. Nope, only fix is to switch ISP's. Frontier. Your Openreach Optical Network Terminator (ONT) which connects your premises to our network has a number of status lights. Its lights should all glow a steady green. If any light is flashing or switched off, select the option which describes its status: The mains is unplugged or there is a problem. The signal shows a full signal, but the network speed is still slow? What does it mean when the ONU indicator keeps flashing? Plug in and light up, showing whether ONU is connected to power, ONU without power connection is useless.