Mobile services greyed out on Family Sharing iphone

We have new 2nd hand phone that came factory reset from a reputable source (iPhone 13).

We have set it up with family sharing via my wife's iPhone.

I have turned off Content and Privacy Restrictions under Screen Time for now.

However, I want to install an eSIM, but the Mobile Service option is greyed out.

The phone never asked me to set up a SIM card when I first set it up.

I have tried rebooting (after update to latest iOS), and resetting network settings, but cannot find a way to install an eSIM (which is from a provider that does specialised aims for kids).