Get ACF repeater within a repeater without using ACF functions.

ACF Post Meta - Repeater chaining.php

<?php

// get post ID
$pid = get_the_ID();

// get our repeater
$repeater = get_post_meta( $pid, 'acf_repeater_slug', true );

if ( !empty($repeater) ) {
  for( $i = 0; $i < $repeater; $i++ ) {

    // get our subsection repeater
    $subsection = get_post_meta( $pid, 'acf_repeater_slug_' . $i . '_acf_sub_repeater', true );
    for( $j = 0; $j < $subsection; $j++ ) {
      
      // get field for subsection repeater
      $acf_field = get_post_meta( $pid, 'acf_repeater_slug_' . $i . '_acf_sub_repeater_' . $j . '_acf_field', true );
      echo $acf_field;
      
    }
  }

}