Check in order whether the following problems are occurring on the RDS DB instance.
Solution: Check the connection mode. The RDS can be accessed only through an ECS in the same VPC.
Solution: Check whether the CPU usage and the number of current connections are normal by using the RDS resource monitoring function. If either of them reaches the maximum, then reboot, disconnect, or change the CPU and memory of the DB instance.
Solution: Reboot the RDS DB instance to see if the problem is resolved. If the problem persists, contact post-sales technical support.