It's an ion cannon beacon, not a small portable ion cannon. With this in mind, it starts raining outside the chamber.
The only problem is rule 3.
"You cannot modify the room or the doorway."
Using an ion cannon from outside would mean that the room would have to be breached.
Think of it like this: The room is a dimension, and the dungeon is another. Nothing exists outside of the room or dungeon, except that people can enter either or be sent to either depending on circumstance.
The construct of the dungeon and the room is indestructible.