RE: FN Keys not working
There are only drivers available for windows 7 and Windows 10 on support.dell.com for the Inspiron 15 7000 series 7559. Typically though the keyboard FN functions are controlled by Quickset where you can try the Windows 10 version here.
If this doesn't work, try taking a look at function key behavior in the BIOS. Hold the shift key down while clicking restart and choose the BIOS, choose the advanced tag and toggle between function key and multimedia key and see if it makes a difference.
Specialize in Laptops, Mobile Devices