Visibility Notify
From Oldunreal-Wiki
Used to fix Network Replication with specific mirrors or WarpZones.
Reason
By default, stuff reflected from mirror (that are normally behind a wall) or seen through WarpZones, are not considered as relevant actors in online games and will appear invisible.
Usage
Basically you place 2 (Actor -> Info ->) VisibilityNotify actors on both ends, set up a collisionradius/height that covers up visible area of the both rooms and give them both same tag.
Here's a picture of it all (stuff within the light green area will be visible online through the portal):
