ALB, EC2 Instance must be in the same subnet?

My Public ELB is connected to Public Subnet A, Public Subnet B. And there is an EC2 instance in Private Subnet C in the same AZ. In the above environment, the EC2 Instance in Private Subnet C is registered in the Target Group of this Public ELB and used well. Here I want to add instances from Private Subnet D (same AZ) to Target Group of this Public ELB. Request Timeout occurs, can you tell me why and how to solve it?