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, if they don't know where it is have them follow the ethernet cable coming out of the WAN port on their router and it will take them to it.
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, or the reboot ONT through the providence portal for that customer.