SQL Server on RDS Issue With Linked Server

I'm testing Linked Server on RDS SQL Server to point to existing SQL Server (non RDS). Test connection throw error with the following message :

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ADDITIONAL INFORMATION:

An exception occurred while executing a Transact-SQL statement or batch. (Microsoft.SqlServer.ConnectionInfo)

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Named Pipes Provider: Could not open a connection to SQL Server [53]. 
OLE DB provider "SQLNCLIThe test connection to the linked server failed.
11" for linked server "GFHSVRHRISDEV01.corp.xyz.co.id" returned message "Login timeout expired".
OLE DB provider "SQLNCLI11" for linked server "GFHSVRHRISDEV01.corp.xyz.co.id" returned message "A network-related or instance-specific error has occurred while establishing a connection to SQL Server. Server is not found or not accessible. Check if instance name is correct and if SQL Server is configured to allow remote connections. For more information see SQL Server Books Online.". (Microsoft SQL Server, Error: 53)

I have validated the network connectivity as well as domain name resolution using EC2, domain resolution works, telnet to port 1433 also open. As this RDS integrated with the Managed Active Directory, I have also checked that the Conditional Forwarding works to lookup target domain.

The target database also already configured to accept remote connection.

What would be the possible root cause for this? I'm looking for any hints to further troubleshoot this issue.