Unable to connect to an AWS RDS read replica of MySQL I'm not able to connect to an AWS RDS MySQL read replica.
$ mysql -h my-db-read-replica.qprdbd0pavr5.us-east-1.rds.amazonaws.com -umyname
ERROR 2003 (HY000): Can't connect to MySQL server on 'my-db-read-replica.qprdbd0pavr5.us-east-1.rds.amazonaws.com' (60)

Here're the configurations I've checked:

The security group allows me to connect from my local computer to the remote RDS.

The Public accessibility  of the read replica is Yes

I can use nmap to detect that port 3306 on the RDS is open.


$ nmap -Pn -p 3306 my-db-read-replica.qprdbd0pavr5.us-east-1.rds.amazonaws.com
Host discovery disabled (-Pn). All addresses will be marked 'up' and scan times will be slower.
Starting Nmap 7.91 ( https://nmap.org ) at 2021-05-18 17:12 CST
Nmap scan report for my-db-read-replica.qprdbd0pavr5.us-east-1.rds.amazonaws.com (3.208.xxx.xxx)
Host is up.
rDNS record for 3.208.xxx.xxx: ec2-3-208-xxx-xxx.compute-1.amazonaws.com

PORT     STATE    SERVICE
3306/tcp filtered mysql

Nmap done: 1 IP address (1 host up) scanned in 6.46 seconds

I've also rebooted the read replica, but I still can't connect to it.
The connection worked normally yesterday.
What could be the reason for failure of the connection?
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.