Account used in AppPool on server 2012 (in DMZ) cannot connect to SQL Server 2014 inside DOMAIN

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IIS

SQL-Server-2012

Help! We are in the process of creating a web server in the DMZ. Let's call it 'X' Web Server. We have a website in the web server which uses integrated security. The website utilizes an Application Pool called 'AppPoolX'

AppPoolX uses a local web server account for the Application Pool Identity (Custom Account)

Web Server X needs to connect to a SQL Server 2012 (or 2014, I forget), let's call it SQL-Y which is INSIDE OUR DOMAIN.

How can the Application Pool Identity account used in AppPoolX access the SQL server if AppPoolX's account is a LOCAL account for Web Server X?

Please help me with this connection issue.

Thank you.