To Nha Notes | Sept. 14, 2022, 6:37 p.m.
To attach Amazon EC2 instances located in a private subnet, create public subnets in the same Availability Zones as the private subnets used by the backend instances. Then, associate the public subnets with your load balancer.
https://aws.amazon.com/premiumsupport/knowledge-center/public-load-balancer-private-ec2/