@b-thanapat You should try more words to expain your scenario in order to make it easier for your friendly neighborhood programmer to help out.
That said. I store the roomid in a cloudstore under security rules so that only the relevant users can access it and use that to join if the room is created. But if everyone leaves the room will be disposed and you cannot rejoin that instance. In case what your are looking for is not the actual room but the room state, I serialize it ad store it as a file blob in the cloud, accessible only by the server. Then when the room is created I download and initialize the store using that state.