Stuck at 900 elo


Learn and apply the most important principles of chess. - (core of my teaching)
Always blunder-check your moves.
Solve tactics in the right way.
Analyze your games.
Study games of strong players.
Learn how to be more psychologically resilient.
Work on your time management skills.
Get a coach if you can.

Yes, but every chess-site is different. And every time-control is different, so being at 900 in Rapid at chess.com means being at the 70th percentile.
At gameknot correspondence-chess I'm rated 1650 classical ELO, which is about the 67th percentile.