“External data sources” are quite peculiar objects. For the data source you created, you need to configure the connection not only in Designer mode, but also in the interactive (runtime) mode.
To do this, go to the “Advanced tools...” menu, find your data source in the object tree, and go through the entire process of connecting it to the database again. If the connection is successful, you should see the data table. Only after that will you be able to use the external data source in the program’s interactive mode…
My general advice is not to use this object in actual work. As far as I understand, it’s more suitable for one-time operations, when you just need to transfer data from one database to another…