Tim_Shakh, you are correct: the word is вошёл, not вошел. Duolingo is extremely careless and they don't care. You pointed out this error one year ago and it has not been corrected yet!!
One problem is that when you click on "Report" for this sentence, it only gives the choices "The audio does not sound correct" and "The Russian sentence is unnatural or has an error", neither of which is an accurate description. It doesn't give "My answer should have been accepted." However, I'm going to choose "The Russian sentence is unnatural or has an error" because it's the better description of the two provided.
In a previous exercise it used вышел в сеть for essentially the same thing - is there a difference and when would you use one vs the other?
Actually, войти (to go in) and выйти (to go out) are opposite words, but in this particular case (becoming online) they both fit.
But if you вышел (go out) you have place из instead в. Когда он вышел из сети? Because Когда он вышел в сеть? = Когда он вошел в сеть?