The most common causes of this are loss of power to the fiber terminal (ONT) or an unplugged network cable. To identify why your fiber internet isn't working, it's important to establish where the connection problem is. In many cases, a fiber connection problem originates from one of the following. When issues like signal loss, slow speeds, or intermittent connectivity arise, systematic troubleshooting is key. This guide will walk you through diagnosing and resolving common fiber network issues efficiently. Again, your terminal device is the hardware piece that connects your home to the Internet. The diagnostic router they had was able to connect over PPPoE and got internet service, so they said their hardware was good and left. My Asus GT-AX11000 running Merlin WRT version 386.