You can’t use get_sub_field() outside of a have_rows() loop.
You can get the value of that sub field directly using get_post_meta().
For example, if your repeater is named “repeater” and the sub field you want to check is named “sub_field” and the row you want to check is the 2nd row then
$value = get_post_meta($post_id, 'repeater_1_sub_field', true);
indexes start a 0 so the second rows index is 1.
This is how repeaters and sub fields are named
"{$repeater_name}_{$index}_{$sub_field_name}"
