I've only played a matter of minutes and already stuck. I've gone through the fire escape where the guard followed me but I'm stuck outside now. I can't figure out how to reopen the door. The metal bar does nothing here.

It's been a while since I've played this, but I don't recall any "trick" or "gotcha" here. You just open the door again and go back inside. You don't need the metal bar or anything else.

Maybe you're not clicking on exactly the right spot on the door?

you don't need it, just right click the door