DBZ: Why did androids 19 and 20 show up to destroy the city instead of 17 and 18 like in trunks time?

Well it's not really fully explained. Future Trunks went back in time to stop the Z-Warriors from dying and save Goku from dying of a virus that had no cure until after then. He also warned them of the Android attack, when and where it would happen.

A bit of a problem is he didn't describe what the Androids looked like. That and Future Trunks later learns what ever he does at the timeline he went too wouldn't change his, being an Alternate Timeline. Cell's the same, from an Alternate Timeline that would be unaffected by any change done in the past.

So even if Cell's destruction in Gero's lab happened in the timeline before Cell fully matured, the Alternate Timeline Cell wouldn't cease to exist.

The idea is a Multiverse theory; when a choice that is made, another worlds/universes appears to follow timelines of those other choices.

There wasn't really an in-depth explanation given for this, but Future Trunk mentioned the possibility that the very fact that he entered their timeline changed some things, even if he seemingly had nothing to do with it. Meaning, even if a time traveler goes back and time and doesn't directly influence something, a change in that timeline could still occur.

The cell from which timeline?

The Cell that was invading the main timeline was from ANOTHER alternate timeline, where Trunks successfully defeated Androids 17 and 18. Since 17 and 18 had already been destroyed in that timeline, Cell had no way to go into his Perfect form, so he killed that timeline's Trunks, stole his time machine, and went back in time to the main timeline.

The cell that was in Gero's lab in the main timeline was, in fact, destroyed by Krillin and Trunks, ensuring that a main-timeline Cell didn't appear in their future.

Its implied that Cell and trunks coming from the future altered the timelines.