Hi @holgerthorup, thanks for reaching out on Vercel Community.
It appears that your RDS instance isn’t publicly accessible given the IP address you’re trying to connect to is 172.16.2.90. The whole 172.16.0.0/12 range is private according to RFC 1918.
In order to connect to it from localhost, you will need to make your RDS instance public. See Working with a DB instance in a VPC - Amazon Relational Database Service.
The same is also the case in Vercel build environments and functions, however it is possible to keep your RDS instance private using Secure Compute and VPC peering.