Window server2012 R2 standard on windows authentication given 410:Unauthorized Access

I have a window server 2012 R2 standard.I have hosted my application in IIS and authentication is windows.I have set the user identity in application pool as a custom identity.I have added the custom user identity as a administrator group and have given full access tothe path of the published project.Since I am admin user to the server so I am able to access the application outside the server.But external users are getting "401 - Unauthorized: Access is denied due to invalid credentials.".I have tried change the provider of windows authentication to NTLM to top then Negotiate. Changed useAppPoolCredentials to true .But none of them work for me.Can someone please help to resolve my issue.

Thanks,

Joy