remap.sh

#!/usr/bin/env bash
# To run at startup:
# sudo defaults write com.apple.loginwindow LoginHook `pwd`/remap.sh
# https://developer.apple.com/library/content/technotes/tn2450/_index.html
CAPS_LOCK="0x700000039"
ESCAPE="0x700000029"
NON_US_BACKSLASH_PLUS_MINUS="0x700000064"
GRAVE_ACCENT_AND_TILDE="0x700000035"
SLASH_PIPE="0x700000031"
ENTER_RETURN="0x700000028"
RIGHT_SHIFT="0x7000000E5" #Do nothing button
FROM="\"HIDKeyboardModifierMappingSrc\""
TO="\"HIDKeyboardModifierMappingDst\""

hidutil property --set "{\"UserKeyMapping\":[
{$FROM: $CAPS_LOCK,                      $TO: $ESCAPE},
{$FROM: $ESCAPE,                         $TO: $RIGHT_SHIFT},
{$FROM: $GRAVE_ACCENT_AND_TILDE,         $TO: $SLASH_PIPE},
{$FROM: $SLASH_PIPE,                     $TO: $ENTER_RETURN},
{$FROM: $NON_US_BACKSLASH_PLUS_MINUS,    $TO: $GRAVE_ACCENT_AND_TILDE}
]}"

Do you know how can i remap the fn key on my external keyboard? Karabiner doesn't detect my fn keystrokes as well.

Hm, don't know how to help you. The table provided in the technotes doesn't have Fn listed