Try this one first (note that while booting, hold shift to get into the recovery menu, then press ‘e’ - if shift doesn’t work, try tapping esc key repeatedly):
1. Passing acpi_backlight=vendor to the kernel at boot
To try this temporarily, on boot at the grub screen, with the Mint option highlighted, press ‘e’ on your keyboard. If the Grub boot menu does not normally show up on boot, press and hold the ‘shift’ key whilst booting until it shows. On this screen, navigate to just before it says ‘quiet splash’ and type
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acpi_backlight=vendor
then press ctrl-x to boot it.
Test your Fn keys once the system boots and post here.