Sitecore -Getting IIS error pages outside the server instead of custom page not found/error page

Recently we faced an issue where custom page redirect on page not found/access denied was working fine inside the server but was showing IIS pages outside the server

One of my colleague Sufal came up with the fix for this which is as below

In web.config  - Add  <httpErrors errorMode="Detailed" /> just above </system.webServer> node.

or you can do the same directly in IIS -> Select your site -> error pages -> Edit feature settting -> Select detailed errors