When the glowing door is unlocked, return to trash heap. The NPC will guide you to the next step on the path to finding it. In the room in the mansion that seems like it obviously has a secret, there’s a switch on an object to interact with. (I’m not sure how you’re really meant to find it other than randomly poking everything.)
Make sure you don’t save out side the cyber world when you leave it. You can’t return to the cyberworld after you save in your town.