If your device shipped with Windows 10, install the (successor) from the official support page.
If your Fn keys are not working after a clean Windows 10 installation, follow these steps to restore functionality: FAQ: What is Toshiba Function Key? - Support - Dynabook 24 Nov 2014 — toshiba function key utility windows 10 64 bit
Windows 10’s Fast Startup can block hardware utilities from loading properly. If your device shipped with Windows 10, install
Restart your laptop.