Can't change screen time passcode on child's iPhone

So I've shared the screen time passcode with my little brother (kid whose phone is in question), thinking I could easily change it.

However now I cannot change it on my Macbook or directly on his iPhone.

As a background:

I had an iPhone XR and when I got a 13 mini ,I gave him the XR

Then I got a Samsung Phone and gave him the 13 mini (so I have no iPhones on my AppleID anymore)

I still have a Macbook with my AppleID which is set as the guardian of his AppleID.

Things I've looked for:

Change screen time passcode/disable screen time passcode on his iPhone (does not exist)

Disable screen time settings, so I can reenable (does not exist)

General screen time settings for his iPhone on my Macbook.

If anyone has encountered something similar or knows how to fix it, please let me know.

Cheers!