Bloqué en 3g

Just because you swapped your SIM card, doesn't mean your account on your new iPhone has been provisioned correctly. Contact your carrier directly for help. I know you wrote this in French, but where in the world are you using your new iPhone?

If you've really sought help from your carrier and they can't help you, then schedule an appointment at your Apple Store Genius Bar and let Apple test your phone. I truly doubt there's an issue with the phone as you're getting cellular reception. I still believe the issue is with your carrier. But wherever the issue is, this is a user to user only forum and we can't test your phone for you.

Also, this is the English speaking community. You may want to post your issue in the French Speaking Community --> Communauté d’assistance Apple officielle

3G or 5G what is the problem?

Also if you changed your SIM, do this

Change of number

1.    Settings, Apps, Messages,

2.    turn off iMessage   

3.    Settings, Apps, Facetime,

4.    Turn off Facetime,   

5.    Power down the phone

6.    Turn on the phone again 

7.    Settings, Apps, Messages, turn on iMessage,   

8.    Settings, Apps, Facetime, turn on Facetime

 3G ou 5G, quel est le problème ?

Si vous avez changé de carte SIM, procédez comme suit :

Changement de numéro :

1.    Paramètres, Applications, Messages

2.    Désactivez iMessage

3.    Paramètres, Applications, FaceTime

4.    Désactivez FaceTime

5.    Éteignez le téléphone

6.    Rallumez le téléphone

7.    Paramètres, Applications, Messages, activez iMessage

8.    Paramètres, Applications, FaceTime, activez FaceTime