AWS project: set RDS in private RDS to start working (with API gateway, Lambda and Cognito)

Job ID: 33028808

Budget: $250 – $750 USD

I have a project on AWS which consists of:
API gateway
Lambda functions
RDS (MySQL)
Cognito
There is a EC2 instance that runs VPN
Lambda functions in VPN
The project was running when RDS was public, but when I made RDS private inside a VPC I could not make it work. I got lost and probably made VPCs a complete mess.
I just need the project to start working (with private RDS) so the app can again access the system.