I’m not sure of exactly when or how you want this to work, but you should be able to use the following trigger action:
Visibility - Create an initially Enabled visibility modifier for Player 1 (Red) emitting Visibility across (Playable map area)
You change the player (or add additional actions for other layers), change the type of visibility (black mask, fog of war, or visible) and change the region from playable map area to any region or area. Experiment with that trigger and if you still have problems, let us know.