pension refund but no confirmation of exemption

Title might be confusing, not great at summarizing.
So, I’ve been here since August on a working holiday visa.
I speak conversational JP but finance/immigration stuff is difficult for me and when there’s a lot of information at once (no matter the lang) stuff goes over my head and I forget quickly because I’m overwhelmed.

I went to city hall to register my address mid-late August. I signed up for national health insurance I remember that. I don’t remember anything they might have mentioned about pension or signing an exemption application for it. I was signing so many things. I’m also not working rn if that makes any difference.
Anyways, I never received a confirmation letter for MyNumber so I can’t apply for the card, and I never received confirmation of being exempted from pension but did get the bills for health insurance and pension.

I’ve been paying them monthly. Few days ago I get something in the mail to get a refund for Aug/sept pension premiums ? I’m not really sure what to do with it, if I need to write/fill anything in or where I’m supposed to send it.
Has anyone received smth similar and could help me out?

by Careless-Market8483