<form id="changeCustAddr" action="#" method="post" data-url="<?php echo admin_url('/admin-ajax.php'); ?>">
<input type="hidden" name="action" value="cust_address">
<input type="hidden" name="nonce" value="<?php echo wp_create_nonce( 'cust_address_nonce' )?>">
<input type="hidden" id="record" name="record" value="<?php echo (!$newone) ? $rec : $max+1 ?>">
<input type="hidden" id="custno" name="custno" value="<?php echo $custno ?>">

input fields for addresses
Prev, Next and New buttons etc have been removed for clarity

<input type="submit" id="CustAddrDelete" name="delete" class="btn" value="Delete" data-url="<?php echo bloginfo('template_directory') ?>/scripts/deleteCustAddrform.php?cust=<?php echo $custno ?>&record=<?php echo $rec ?>" >
<input type="button"  id="CustAddrNew" name="new" class="btn" value="New" data-url="<?php echo get_page_link( $post->ID ),'?cust=', $custno, '&record=new' ?>" >

<small class="field-msg js-form-submission">Submission in progress, Please wait &hellip;</small>
<small class="field-msg success js-form-success">Details Successfully submitted, thank you!</small>
<small class="field-msg error js-form-error">Could not update database. Please try again.</small>

<input type="submit" id="CustAddrSave" name="save" class="btn btn-blue" value="<?php echo (!$newone) ? 'Save' : 'Add New' ?>" data-action="cust_address">
<input type="button" id="CustAddrClose" name="close" class="btn btn-blue" value="<?php echo (!$newone) ? 'Close' : 'Discard Changes' ?>" data-url="<?php echo get_page_link( get_page_by_title( 'dojo details' )->ID ),'?cust=', $custno, '&record=1' ?>">

document.addEventListener('DOMContentLoaded', function (e) {
const changeCustAddrFrm = document.getElementById('changeCustAddr');
if (changeCustAddrFrm) {
    changeCustAddrFrm.addEventListener('submit', (e) => {
        e.preventDefault();

    let data = {
        custno: changeCustAddrFrm.querySelector('[name="custno"]').value,
        priority: changeCustAddrFrm.querySelector('[name="record"]').value,
        address1: changeCustAddrFrm.querySelector('[name="address1"]').value,
        ... more address lines ...
        postcode: changeCustAddrFrm.querySelector('[name="postcode"]').value,
  }
  // ajax http post request
  let url = changeCustAddrFrm.dataset.url;
  console.log(url);
  let params = new URLSearchParams(new FormData(changeCustAddrFrm));
  console.log(params);
  changeCustAddrFrm.querySelector('.js-form-submission').classList.add('show');

  fetch(url, {
      method: "POST",
      body: params
  }).then(res => res.json())
  .catch(error => {
      resetMessages();
      changeCustAddrFrm.querySelector('.js-form-error').classList.add('show');
  })
  .then(response => {
      resetMessages();

    if (response === 0 || response.status === 'error') {
                changeCustAddrFrm.querySelector('.js-form-error').classList.add('show');
                return;
            }

            changeCustAddrFrm.querySelector('.js-form-success').classList.add('show');
        })
});

public function cust_address() {
    if (! DOING_AJAX || ! check_ajax_referer('cust_address_nonce', 'nonce') ) {
        return $this->return_json( 'error' );
    }
$custno = $_POST['custno'];
$addressno = $_POST['record'];
$max = $_POST['max'];

    $servername = "localhost";
    $username = "uuuu";
    $password = "pppp";
    $dbname = "db";
$mydb = new wpdb( $username, $password, $dbname, $servername );
$table = 'mem_custaddress';

    $data = [
      'Address1'=>sanitize_text_field($_POST['address1']),
      ... other address lines ...
      'Postcode'=>sanitize_text_field($_POST['postcode']),
    ];
    $format = [ '%s', ... , '%s' ];
    $where = [
      'Custno'=>$_POST['custno'],
      'Priority'=>$_POST['record']
    ];
    $where_format = [ '%d', '%d' ];

    if ($addressno <= $max) {
      $result = $mydb->update( $table, $data, $where, $format, $where_format );
    } else {
      $dataInsert =[
        'Custno'=>$_POST['custno'],
    'Priority'=>$_POST['record'],
        'Address1'=>$_POST['Address1'],
        ...
        'Postcode'=>$_POST['Postcode'],
      ];
      $formatInsert = ['%d', '%d', '%s', ... , '%s'];
      $result = $mydb->insert( $table, $dataInsert, $formatInsert );
    }

    if ($result) {
        return $this->return_json( 'success');
        wp_die();
    }
    return $this->return_json( 'error');
    wp_die();
}

public function return_json($status)
{
    $return = array(
        'status' => $status
    );
    wp_send_json($return);
    wp_die();
}