So Iblis is a jinn because he disobeyed Allah and because he was created from fire. When Allah commanded Al-Malaika to prostrate to Adam, Iblis disobeyed and thus became from the disbelievers. Thus, he cannot remain in the Jannah. So how can Iblis seduce Adam(as), who is in the Jannah, into eating from the forbidden tree? What I mean is that Iblis seduce us, humans and jinn, into committing sins because we are on the same world. Iblis is not in Jannah and is on earth with us, which is not like in the case of Adam(as).
- He re-entered Jannah, as expulsion does not preclude temporary or unwelcome entry. Some traditions say that he entered in the mouth or form of a serpent in order to get past the guard.
- He did so without physically entering Jannah. For example he spoken from outside its gate.