@huanaco That usually means the phone is locked. Which J3 do you have and where was it purchased?
The fastest way to isolate the problem is to try your activated Publc Mobile SIM in an known unlocked phone and an operating SIM (but not your last carrier) in yours.
You seem bilingual. In future you will find responses are generally faster in the English forum.
>>> ALERT: I am not a CSA. Je ne suis pas un Agent du soutien à la clientèle.
