Unable to connect to an AWS RDS read replica of MySQL
The size of my MySQL writer instance is db.t3.medium, and that of my read replica was db.t3.small.
After upgrading my read replica to db.t3.medium, I'm able to connect to the read replica.
I don't know how to explain this phenomenon.
This page says

It's a best practice that all instances in a cluster are the same size to avoid any workload overload on the reader instances.

But my read replica's workload is not high.