Unable to set up DMS endpoint (Both source and target failing) - Unsure why

Hello, I am playing around on AWS to build my knowledge. I have an EC2 Instance with Postgres/psql on it and a postgres database. I am trying to migrate this to RDS using DMS, I have set up a the postgres RDS instance and set up the replication instance and they both have the same security group attached. The EC2 (source) database, RDS (target) database and replication instance are all within the same VPC but each in a different subnet. The security group inbound rule for the RDS & replication instance is PostgreSQL, with the source as that same security group. The security group inbound rule for the EC2 database has PostgreSQL with the source as the security group of the replication instance. I have followed this: https://explore.skillbuilder.aws/learn/course/36/play/77/migrating-from-postgresql-to-amazon-rds

But still when I test out the DMS endpoint when I'm setting it up, it fails. The target endpoint is failing stating there is no pg_hba.conf entry for the replication instance IP. The source endpoint is failing with a connection timeout. I have tried manhy troubleshooting options, researched websites but had no luck. I'm not sure why they are failing, any help appreciated or even a point in the right direction. Thanks.