Icloud login issue - unknown error - iPhone

Hi,

I have an issue with my daughter devices / include settings.

She has 3 devices;

Iphone SE

Ipad air

mac book air

The lates two are signed in to include but we cannot connect I phone. When trying to do so I get message unknown error. Option to sign in is only availaible the I restart the phone as otherwise from settings the "Apple ID , I cloud, Media..." function is grey out.

I was under impression that it can be related to her old phone number still being linked to account so i have manage to change her number in phone settings (my number) to current...messages and face time are working but can not adjust any settings in it (again grey out).

When I log in to Appel ID via inrenet browser the old number is still showing there...its telling me to go to i message and remove it but as its all grey out so I cannot do it. Her I phone do not appeer as connected divice as not sign in.

Im not sure how to correct it all.

Appreciate your help and advice.

Thanks

Adam