Problem with Nintendo Switch Online on multiple accounts?

My brother recently bought us the family offer from Nintendo Switch Online, so we can play online on multiple accounts. Everything works fine on his account, but not mine. Does anyone know how to use this on another page, or whether we still have something to stop?

One guess I still might not work because my page is signed in with another Nintendo account, even though it's the same console.

If anyone knows that, I ask for clarification.

You have to merge the accounts into a family group

https://accounts.nintendo.com/family

Thank you. ^^