How can I connect my RDS instance to my backend for free?

Having a publicly accessible database can really open up security risks, so it's usually not the best practice. Personally, I think it might be a good idea to move your backend service to AWS itself instead of using Hostinger. This way, you could keep everything within AWS and avoid extra charges like public IP fees.

I found this helpful article that explains why you're being charged for that public IPv4 with the RDS free tier. You can check it out here: https://go.aws/3T7iPSS. Also, if you're looking for a rundown of what's included in the Free Tier for RDS, take a look here: https://go.aws/3Teq2QY.

Here are a bunch of useful links to explore:
- https://aws.amazon.com/products/databases/
- https://aws.amazon.com/rds/
- https://aws.amazon.com/dynamodb/
- https://aws.amazon.com/aurora/
- https://aws.amazon.com/redshift/
- https://aws.amazon.com/documentdb/
- https://aws.amazon.com/neptune/

And, maybe check Reddit's search for more insights on databases in the Free Tier!

If you're set on using Hostinger, it might be tough to avoid charges because AWS generally requires at least a public IP to connect from external services. Upscaling to a VPS on Hostinger could help, but then you'd need to figure out access to that DB.