ElastiCache clusters should not use the default subnet group

How to fix it

Set at creation: fixing it means replacing the resource.

Move the cluster onto a subnet group of your own instead of the one called default. A cluster's subnet group is fixed at creation and no modify operation accepts a different one, so this is a replacement: create the subnet group, back the cluster up, restore into a new cluster in it, repoint every client at the new endpoint, then delete the old one. Memcached has no snapshots, so it is recreated empty. A member of a replication group cannot be moved alone; replace the group.

In the console

ElastiCache → Subnet groups → Create subnet group, then the Subnet group setting on the replacement cache

With Terraform

resource "aws_elasticache_cluster" "this" {
  cluster_id        = var.cluster_id
  engine            = var.engine
  subnet_group_name = aws_elasticache_subnet_group.custom.name
}