Why are power cells unavailable for the Ancient Armory side quest after finishing the main quest

This guide here lays out where all five of the cells are:

1. The Underground Bunker in the prologue. Once you are adult Aloy, go back to this locations. The cell is located on the other side of some stalactites and stalagmites inside of the bunker.

2. The All-Mother Temple. After or during the Womb of the Mountain quest, this cell can be found through two doors and a vent after you awake.

3. Makers End. The guide recommends getting this one during the quest that is also named Makers End, since you will have climbed the tower already. Once at the top of the tower on the rooftop, turn around and see a bronze looking spire. Climb the spire, and the power cell is on top of it.

4. The Grave-Hoard. You can get this one either during or after the quest called The Grave-Hoard. Enter the ruins, solve the puzzles, climb up the ledge and open the door in front of you. The power cell will be in front of you along with some supplies.

5. Inside the GAIA Prime ruins. This is towards the end of the main story line quests. You can get it once you begin The Mountain That Fell quest. After proceeding through the ruins, you will reach a zipline outside. After dropping off of the zipline, turn around. You'll see a handhold on a rock that you can climb. After climbing, you will see a purple looking entrance to a small cave. At the end of the cave, is the power cell.

_{All image links are from the above linked guide}

It depends on the area you are going to face.
I bought and invested in Protector armor, which works well against melee attacks and it's useful in different cases. I found long range protections quite useless.

Surely it's good to have fire-protecting and ice-protecting armors in late game. Elemental defences are situational, when facing specific machines they are a must-have in harder difficulties. As a side note, I prefer to maximize a singular element, instead of using more general mods. It's better to do something at best.

As @Mad Scientist is asking, the DLC area is strongly based on ice element, but fire-based machines are pretty common, too.