If you exit the hotel near the cafe / plaza (near where the Starbucks used to be before it moved) and head toward the new Starbucks and towards Roppongi, you will see an escalator that will take you underground and drop you off near a Muji, 7-11 and some other shops on your way to the metro. There is only some brief exposure to the outside on this route.
In the reverse, coming back to the hotel from the metro, if you pass those shops mentioned above and don't take the escalator up to the plaza but instead head towards the office building entrance, turn left when you reach the office doors. This will lead you to a passage way to the RC bell desk and elevators.
Since the porte-cochère for taxis and the ground entrance to the mall are on different levels it can be somewhat confusing but it offers a couple different routes to and from the hotel when the mall is closed.