To unlock the Rebuilding DC achievement in The Division 2, you will need to fully upgrade The Theater and Campus settlements to level four. Other settlements in the game can't be upgraded, so you don't have to worry about them.
You will visit both of these settlements naturally through story progression. To upgrade them, you will need to complete main story mission and side missions that are given out by NPCs at each settlement.
You can view the level of a settlement near the camp leader. It will be indicated by a green circle with a number in the middle. The outside of the circle will begin to fill as you make progress towards the next upgrade level. Once the outside of the circle is full and the number 4 is displayed in both The Theater and Campus settlements, the achievement will unlock.