So, you are saying that you have a fibre connection to the Internet and that includes a device known as an ONT?
Please remember that the AirPort Extreme was designed to connect to normal cable and DSL modems, not ONTs. In other words, there are no specific settings on the AirPort Extreme that would support a fibre connection.
So, it may or may not be possible for you to connect the AirPort Extreme to your current fibre Internet connection.
You can try factory resetting the AirPort Extreme and the try to set it up again a few more times, but if you continue to see the same results, the AirPort Extreme is just not going to work with your fibre connection.