There are two places you can go to trigger Act 2, either by going into the elevator in Grymforge or continuing into the Mountain Pass after meeting/fighting the Githyanki.

There’s a popup in each area to be sure you’ve finished what you want to, as moving to either area progresses the story and quests, and some stuff you don’t do is left undone.

There’s a similar popup between Acts 2 and 3, though there’s only one way to go for that and once you do, you can’t fast travel back to anywhere in Acts 1 or 2.

ACT 2 SPOILER

I didn’t realize until talking with friends that you could win the abduction fight at the Harper camp in Act 2. If you don’t kill the baddies in time, the safe bubble collapses, and everyone in the Harper camp gets shadow cursed, and you have to fight your way through them. It’s genuinely heartbreaking and horrifying.

Yeah, if I post something here, I’m posting it because I want other people to see it. That’s kinda the whole point of reddit-style social media.