I disabled Secure Boot in BIOS and reinstalled Ubuntu 18.10. I followed the steps described in the following link to modify the grub. However, after executing sudo update-grub and rebooted the system, I still cannot change the brightness using the function keys.
Ok, I've figured it out for me, at least with ubuntu 18.04 as of 2/11/2019.
Install ubuntu nvidia drivers:
# use auto-install (as mentioned by @Ibrahim) sudo ubuntu-drivers autoinstall
GRUB_CMDLINE_LINUX_DEFAULTentry in the
/etc/default/grubto the following:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash nvidia.NVreg_EnableBacklightHandler=1"
Update grub configuration after edit and save: