Most issues on a fiber connection are going to be with the home network. It is very rare for a fiber connection to have an issue unless there is an outage, so be sure to quickly identify the exact issue and confirm if their modem can be contacted.
These calls will most likely require educating the customer on how Wi-Fi works, and best practices for a home network.
Why are they calling? What is the root issue?
The fiber modem is usually located in the furnace room or garage. Inside their home is a “Zhone” brand modem, usually looking like the pictures below.
This is the device the fiber connects into and should rarely, if ever, be rebooted. The device may be a different model but will operate the same way.
The “WAN” Port is labeled GE1 on this device. The other ports are disabled and will not deliver internet service.
The customer will never need to change anything on this device, it's just an ONT. If rebooting is necessary, use the black power cable.