You can also create an S3 endpoint in the Datomic Cloud VPC instead of creating separate subnets:
aws ec2 create-vpc-endpoint --vpc-id <DatomicCloud VPC id> --service-name com.amazonaws.us-east-1.s3 --route-table-ids <DatomicCloudRouteTable id>
To find your and you will want to review the tagged resources under the VPC and Route Table consoles respectively.
See the following docs on finding resources by tags:
https://docs.datomic.com/cloud/operation/monitoring.html#tags