That field is on page three which means your code is running after the $_POST is populated so the choice isSelected property isn’t used, that property would only be used for fields on page one. The plugin is using the contents of the $_POST to determine which choices are selected so you’ll need to populate it instead e.g.
$_POST['input_1_1'] = 'the choice value';
You would replace 1_1 with your field and input numbers.