I've been using the n-back protocol for over a year and it's definitely improved my cognitive abilities. I don't know if my IQ has increased, but on days where I use this protocol(20-25 minutes) I see a marked improvement in the clarity and vibrancy of my thoughts- put another way this protocol's biggest benefit is it's effectiveness in clearing the mental fog in my head. I've tried experimenting with other protocols(solving crossword/chess puzzles, meditation,etc..)but they were not nearly as effective as n-back.
I've actually built my own variation of n-back(triple n-back), where you're tested on the colour of the stimulus in addition to the sound and position-so it's much more cognitively demanding. If you're interested, you can check it out here: https://mind-workout.pages.dev