You will probably need to port forward which ever port your RocketChat server is listening to on the pi to an external port on your router.
If as you say its installed and accessible form your local network then you must already know which port it listens on, this is the port you need to port forward to allow internet access.
Once I have forwarded the port (I know the port number and how to forward so I have set this up on the router) how do I verify that I can access it externally (from outside my local network)?
Thank you for your help with this. It is very much appreciated
From pc on another network or possibly a 3g connection on a smart phone,
you need to install the same app/program you were using on the pc on your local network.
Then in this app/program you replace the local address and port you were using to connect to the server with you routers wan address and the port you forwarded..
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