Put a task inside the Player Controller that reads the player pawn coordinate, and then applies a change to the sun actor.
You’d want to do something to find the sun actor when you event begin play in the controller, and you probably only need to run this task every 2 seconds or whatever, depending on how fast the player travels and how far the sun must move.
You also need to set the sun to “movable” and turn off static/pre-computed lighting in the level.