너 is informal. It can be seen as very rude to use it to someone you speak formally with, so it’s a no-no. Commonly you don’t call people ”you” in Korean, you can use either their name, title, or just omit the pronoun. Oftentimes it is easy to understand that you are referring to the other person based on context even if you don’t say ”you”.
To adress someone in formal language, you can say their name (followed by 씨, for example 석진 씨).
From what I’ve gathered, 당신 isn’t used very often so it’s better to call people you speak formally with by name or title, or just without a pronoun if it works in the context.