In Remnant: From the Ashes’ DLC Subject 2923, you must find the key to gain access to the maintenance access to turn on the reactor. Before you can activate it, there are root obstructions preventing the reactor from working. The obstructions are the same root obstacles that were preventing you from progressing further into the facility, such as the one right before the medical bay, or the one in the lower levels. Much like those, you need to interact with the glowing one to enter the other world and shoot them.
There are three obstructions you need to destroy. You can find one on the top catwalk, the middle catwalk right next to where you enter the other world, and then the one on the lower levels. You can take the middle floor one out first. After that, it’s probably better you rush down to the lower level to destroy that one. There’s a large spectral root creature patrolling near it, so the quicker you take out that one, the less time you have to deal with it. The final one on the top floor is on the right side of the catwalk. Luckily, there are no monsters constantly patrolling it, save for the one on the middle level.
After you take out all three obstructions, return to the main reactor’s computer on the middle floor to turn it on. You should see a cutscene as the motors lower, and the facility powers on. The last thing you need to do is destroy the root obstruction to the computer’s right to enter the research area.