I apologize if I haven't searched enough in looking for answer to my question.
After successful submitting, radio buttons are cleared (empty) - there is no default ("on") state, as demo form shows on plugin's site.
If user has submitted data - but made a mistake and wants to correct wrong entry and send the "same" form again - he/she might overlook that radio-buttons are empty (i.e. page is not reloaded).
How to "refresh" the default value for radio-buttons after submitting?