Get the link based on the radio field
$resource_link_type = get_sub_field('resource_link_type');
if ($resource_link_type == 'internal') {
$link = get_sub_field('resource_internal_link');
} else {
$link = get_sub_field('resource_external_link');
}
then show the link
<a href="<?php echo $internal_link; ?>">