RDS Subnet Association

Check the endpoint of the DB instance with the "nslookup" command to see the private IP address.
It can also be determined from its IP address.
RDS creates a network interface.
By checking EC2→Network Interface, you can see that the RDS network interface has been created, and you can also identify the subnet from there.

The following documents may be helpful in moving subnets.
https://repost.aws/knowledge-center/rds-move-to-private-subnet