When you place radio buttons in a button group panel, the panel disables the radio buttons' callback functions and replaces them with the panel's SelectionChangeFcn.
In your case, you can do away with your second button entirely and don't need to define a SelectionChangeFcn for the button group panel. Even without defining that function, the button group panel will allow only one radio button to be selected at a time and will update the SelectedObject property every time a new radio button is selected.
You can use your third button to read the SelectedObject property, determine which radio button is selected, and to plot your data.
The SelectedObject property is a property of the button group panel. The switch case in your code should properly identify the button, as long as the four radio buttons have the correct tag and no other buttons (of any sort) are located within the button group panel.
I think your issue was that your first button included the code to plot your data and needed to wait on the selection. Personally, I would move the plotting function to the Callback of the third button. You can also use the uiwait() function to pause the code for the first button until the third button determines the appropriate sample rate and calls uiresume().
To be clear about which button is which:
The first button is the one that loads the file. The second button checks the sampling rate selected via radio button (if it is used). The third button triggers the plot.
Comment on my answer if you need clarification or have questions. I'll check it periodically.