Hey there! Navigate to your profile and click edit. About half way down the page you'll see a section called "Site Settings." 2/3 of the way down in that box you'll see a setting called "Show the private chat bar while on mobile device." Change that from hide to show.
If you were curious about the chatroom specifically, there is also a setting below the one I described that will allow you to use the full screen website chat, instead of the mobile chat. Let me know if you have any questions