How can a 400 elo player get good at tactics and become advanced or maybe even a master?
The most important step is...learning to calculate/analyse/evaluate.
You don't do this by just looking at a bunch of tactical puzzles. You are nowhere near ready for spending a lot of time on that - that is more along the lines of 'staying sharp' - being on the lookout for specific things to calculate/solve.
I've seen too many players start out by solving lots of puzzles...and they are unable to PLAY the game well. Nice 'puzzle ratings'...but they can't string together the bunch of moves that come in between.
PLAYING a game well is all about being able to walking into uncertainty...looking a couple of moves ahead for yourself and your opponent, all in your head and developing your ability to assess the end points of your calculations.
Yes, study some...a decent and short book on middlegames, a good book on basic endgames...a simple opening...simple tactics; then play fairly slow games (walking into uncertainty), developing your ability to bring all that together and...the rest will come. You will find areas that come to you naturally and others that you need to work on. But you will not find that solving tactical puzzles.
A good foundation and then hard work will take you as far as you let it.
In fact, the last 4 games I played were all settled in the opening. This is another example:
Unfortunately, I forgot about the time and lost on time, but that's not relevant for this discussion. My point here is that my opponent lost a bishop in the opening (in a way that I have seen many times in this opening), and in the rest of the game there are no tactic opportunities to recover from that loss. No matter how much you analyze the game, I don't think you will find anything.