Recently I have faced the similar issue, but when I have followed the Rocko method, the link is broken. So it was not making it download the linux-firmware.
Also I have tried to do: sudo apt install firmware-realtek, still it didn't worked out.
After searching it online, found a new link of git.
Steps to follow:
Download the linux-firmware:
$ git clone git://git.kernel.org/pub/scm/linux/kernel/git/firmware/linux-firmware.git
copy the missing firmware to /lib/firmware:
$ sudo cp -r linux-firmware/rtl_nic/ /lib/firmware/
$ sudo cp -r linux-firmware/i915 /lib/firmware/ (In my case i915 was also missing)
update the initramfs:
$ sudo update-initramfs -u
Note: After update, if still see that, some of the firmware are missing, then search it in linux-firmware and copy those folders into /lib/firmware.