Use X-Forwarded-For HTTP Header to Get Visitor's Real IP Address.

wp-config.php

/**
 * Use X-Forwarded-For HTTP Header to Get Visitor's Real IP Address.
 *
 * @link https://wordpress.org/support/topic/where-to-add-the-x-forwarded-for-code-in-wp-config-php/
 */
if (isset($_SERVER['HTTP_X_FORWARDED_FOR'])) {
    $Arr_http_x_headers = explode(',', $_SERVER['HTTP_X_FORWARDED_FOR']);
    $_SERVER['REMOTE_ADDR'] = $Arr_http_x_headers[0];
}