Do you have access to the fiber modem?
The fiber modem is a small device (black or white depending on the model) set up between the fiber outlet and your Internet Box/b-box. Several models are available (see the image above).
It is connected to the red WAN port on the Internet Box/b-box by an Ethernet cable (see the image below).
If you have access to the fiber modem, reboot it: simply switch it off by pressing the power button, wait a few seconds, then switch it on again.
Do you have access to the fiber modem?