Yeah sorry.
Found the solution.
Wasn't about the sewer.
But I could just walk over the platform above that door.
For yome reason I thought that the background object was olid wall (don't ask me why).
Anyways you can go around that wall by going above.
I thought the outside of that building's wall looked like a solid wall as well, so I added the sign saying come own in. However, you can blow up the sign.... :-) But yeah there's a ladder to get over that wall, but I thought you were meaning to get into the room behind the wall, which is through the sewer.