PostgreSQL supports logical backups. You can use the pg_dump logical backup function to export backup files and then import them to RDS using psql.
You can access RDS through an EIP or ECS.
For details about how to create and connect to an ECS, see section How Can I Create and Connect to an ECS?
For details, see section How Can I Install the PostgreSQL Client?
The PostgreSQL client version must be the same as the version of RDS for PostgreSQL. The PostgreSQL database or client will provide pg_dump and psql.