About ChatClient.PublicChannels dictionnary and chat history

I am trying to view chat history (the messages of other user before the current user subcribed into a channel). From Chat Intro I saw that all messages are stored within ChatClient.PublicChannels, but when I tested, it only stored the messages from the time that current user subcribed (I chat in other devices before connected and only see the messages from the moment I connected (subcribed OnConnected). So did I make some mistake or this is the limitation of the dictionary? How can I do to catch the messages from other user even before I connected?