A new phone works out of the box, but a few settings affect security, backups and everyday convenience.
Screen lock and biometrics
Set a PIN or password, add biometrics and check whether lock-screen notifications reveal too much personal information.
Backups
Make sure photos, contacts and important data are backed up to your preferred cloud or another reliable destination.
Device finding
Enable Find My iPhone or Find My Device before you lose the phone, which is when setup becomes inconveniently late.
App permissions
Review access to microphone, camera, contacts and location. Keep permissions only where they are necessary for a clear feature.
Updates and security
System and app updates patch known vulnerabilities. Enable automatic updates or check for them regularly.
Bottom line
Keep the solution simple: start with the most likely cause, change one factor at a time, and observe the result. That makes it much easier to identify what actually works in your situation.