Connect to Oracle Instance on Amazon RDS

When I try to connect to the instance that I just started on Amazon, I get "ORA-12170: TNS:Connect timeout occurred".

$ sqlplus adminxxx/abcxxxx@xxxxx.ctbxjhstgq8o.us-east-1.rds.amazonaws.com:1521/orcl

SQL*Plus: Release 12.1.0.2.0 Production on Thu Aug 3 12:18:04 2017

Copyright (c) 1982, 2014, Oracle.  All rights reserved.

ERROR:
ORA-12170: TNS:Connect timeout occurred


Enter user-name:

I suspect it has to to with the security group. I click the security group

Then I add port 1521 to the inbound rules.

I save the setting and am able to connect to the instance from my laptop.

$ sqlplus adminxxx/abcxxxx@xxxxx.ctbxjhstgq8o.us-east-1.rds.amazonaws.com:1521/orcl

SQL*Plus: Release 12.1.0.2.0 Production on Thu Aug 3 15:20:17 2017

Copyright (c) 1982, 2014, Oracle.  All rights reserved.

Last Successful login time: Thu Aug 03 2017 15:18:16 -04:00

Connected to:
Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit Production
With the Partitioning, OLAP, Advanced Analytics and Real Application Testing options

SQL> select count(1) from user_tables;

  COUNT(1)
----------
         0