Server outside of AWS VPC how to join the AWS simple AD?

Question

I m starting to explore the AWS simple AD services.

I have AWS simple AD setup in my AWS VPC, I have one window and one Ubuntu EC2 instances setup in this VPC as well.

I use this Windows server to create users and groups, and it also works well for my EC2 instance to join the Domain I setup in the Simple AD through realm and sssd.

Since when I launch these 2 instances, I can choose "Domain join directory" under the Advanced details, so I guess this is one of the reasons why my instances can discover the AD. Reference Link: https://docs.aws.amazon.com/directoryservice/latest/admin-guide/simple_ad_launching_instance.html https://docs.aws.amazon.com/directoryservice/latest/admin-guide/simple_ad_join_linux_instance.html

However, I don't know how to let my ubunut server which create outside of AWS to discover the domain I setup on the simple AD.

ubuntu@hostname:~# realm discover example.com 
realm: No such realm found: example.com

I think first I need to make sure the server outside of AWS should communicate to the VPC where the AD is located.

But I m still not sure what's will be the whole steps to make it works.

Thanks if anyone can provide some thoughts or ideas.