Fast forward, using the markings on the smd led I was able to determine the cathode & anadode and everything worked.
However, when I try to use my multimeter (a cheap Tacklife) diode test setting to double-check cathod/anode, the screen never changes nor does the led light up. I tested other diodes (not leds) with that setting and it works.
I have no idea exactly what kind of leds they are. Is it "normal" that some leds can't be tested by some multimeters? I don't have any other smd leds handy, nor another multimeter.