In the interest of platform stability, Nagios Log Server uses previous versions of ELK software which may not conform to the default documentation that you reach from a search engine.
As of Nagios Log Server 2.1, here are the documentation links for the correct versions of the backend software:
Article ID: 859
Created On: Wed, Sep 18, 2019 at 3:06 PM
Last Updated On: Wed, Sep 18, 2019 at 3:10 PM
Authored by: swolf
Online URL: https://support.nagios.com/kb/article/nagios-log-server-elk-documentation-859.html