Apple - How do I make function keys work for an external keyboard on MBP with Touch Bar

You can check the following checkbox via System Preferences > Keyboard (default tab):

In karabiner-elements Preferences, in the Function Keys tab, there is an option at the bottom, "Use all F-keys as standard function keys".

Karabiner-Elements will also allow you to bind a key that exists on your external keyboard to Fn, like Caps Lock or PrtSc under the Simple Modifications tab.

You can combine these two, so you can still adjust brightness with your F-keys by holding down the combination of Fn+F1/Fn+F2, where Fn is whatever key you mapped it to.