Capital gains taxes as a student?

Hello, I could use some advice if anybody is able to help!

Does anyone know how foreign capital gains taxes work as a non-permanent tax resident? If I have made reinvestments in my foreign bank account that triggers capital gains, do I need to report these for taxes even if I’m just a student?

For more details, I’ve been a student here for about 3 years, and I had a small amount of capital gains in my US investing account. I haven’t sent any money to Japan from that account specifically, but I obviously have used my other savings accounts to pay for living costs. Since I plan to start working here next year, I want to make sure I didn’t make any tax mistakes.

If I need to report these gains and pay taxes, can I do that even a year late? Is there somewhere or someone I should go to for help?