aops
For OP: I think the problem with your question is that "math" is too broad, e.g. Math Overflow is for professional mathematicians, things like AOPS are very specific and most online communities are about what you would call "recreational" math (think 3blue1brown or standupmaths), likely far below what you want.
But yeah, roughly speaking you need to at least be a math PhD student who have passed your quals to be at the entry level of MathOverflow.