To restore power to the bunker, Nate proceeded alone by climbing across a series of pipes, looking for a way inside the bunker. Once inside, Sully tried contacting him on his walkie-talkie, but was unable to establish a proper signal. As Nate proceeded through the bunker, he was curious as to what happened here, as he came across a trashed room. Shortly after, he was attacked by descendants that appeared from around corners. Using an MP40 submachine gun, he was able to protect himself as he made his way to the generator room, where he managed to restore power to the bunker.
You begin by making a short leap to the yellow pipe. From here, climb, shimmy, and leap across the walls using the pipes, and a chain at some point. Although some pipes will partially break off, they will not collapse. Eventually you will reach a platform and have to make one more jump to enter a staircase, until you reach a larger room with a film recorder. Here, you can grab an MP40 submachine gun and load up on ammo, then head out the next doorway on the left end, shortly after encountering two descendants.
Either aim and take them out from afar, or use blind fire when they get close, but keep pressing through the corridors, making sure to reload and restock your ammo after every encounter. Head left at the end of the hallway (the right path is a dead end with more enemies), until you come across another adjacent passage on the left that leads directly to the generator room. Kill any descendants you come across and restock in the generator room, then approach the one generator with a winch (second from the left). Follow the on-screen prompts until the power is restored, immediately prompting the start of the next chapter "Unwelcome Guests".