You should be able to walk through the door as if it’s not there (because it isn’t), but getting the right angle to click on the other side can be tricky, so you may have to fiddle around with the camera a bit.
Ah, yes, thanks.
I had tried "walking" the camera through the door like I'd seen on a few YouTube videos and only saw black and assumed the area was un-clickable. Turns out I can do this and click somewhere in the black area and it can be the floor on the other side, and I then walk through. I had best luck turning the camera 90 degrees away from the door and strafing left with "A".
On the YouTube video I saw the area behind the door was rendered normally, and it was over a year old, so I assumed that a bug had been fixed and some other more in-game obvious way to achieve this had been implemented.