I found this door at the entrance of the Sehwa Hospital, I looked for guides and tutorials on how to enter there but nobody specified the way. Here I will leave you the way to get to this door and get the achievement.
The door has green lights at the top, those lights represent each “antenna” that we activate throughout the game. They are something similar to a wifi antenna that are scattered throughout the game, in total there are 10.
In my case I did it from the last save point, but if you activate all the “antennas” you can go earlier. From the Sehwa Gym we go to the portal that takes us to the Dokkaebi Market, From the market we take the portal that takes us to the Train Station exiting on the lower floor. Once there we must go up to the corridor of the control room and run to the left to go down where we get the key to the station at the guard, we run to the right until we see an open elevator and there we will have a portal that will take us to the Sehwa Hospital .
Finally, to get to the bunker from sector C, you must go up and enter the B3/C3 exit that leads to the ambulance exit. Here you must run to Reception and go down the last staircase, that will leave us in part B2/A2, running to the left we will reach the underground corridor where the Yougho Bunker.