The Japanese SIM is not working on my iPhone 16

I bought my son an iPhone 16 directly from Apple in the UK. He is now in Japan and has a physical SIM card from Mobal, a japanese provider, but it is not working. The phone is unlocked. What else can we try to get it working. The sim does work in an iPhone 14 so the sim is ok.

[Re-Titled by Moderator]

Original Title: Using iPhone 16 with a japanese sim