K810FnLock lets you set the Fn Lock on the Logitech K810 Keyboard. Even though this function is available on Logitech SetPoint software, a lot of people have trouble getting SetPoint working correctly, so this is an alternative.
You can use the provided BAT files or manually enter the mode you want via command line:
Usage: K810FnLock <-on|-off>
I used the research done by Mario Scholz and used a lot of code from Julian Eberius's repository, k811fn, which uses the HIDAPI library.