Tableau Online cannot see inside your network to connect to your local SQL Server, but you have a couple options:
Extract the data, and refresh it on your local machine
Use Tableau Bridge to set up a live connection. Tableau Bridge is installed in your network and connects to SQL Server, then shares the connection with Tableau Online to keep your data secure:
Bridge can also be used to refresh your extracts automatically.
Thanks Carisa - this is the route i'll go down!