If you mean the fault light bottom right then it is the Battery Back Up rechargeable batteries. It’s no longer supported by Openreach and when they go faulty they usually just remove them. (Unplug the power and plug it straight to the ONT on the RH picture).
If you want to keep the ONT, it lifts out of the box, pop off the cover and replace the AA rechargeable batteries with new ones of at least 2000mah or more, then pop it back in to place.
