The standard install does not use nginx installed on the system and if you have nginx installed on the host, Discourse will not have access to ports 08 and 443. By installing nginx on the system, you have prevented Discourse from working in its standard configuration.
This has nothing to do with Amazon. It’s just Linux system administration.
(It can be appropriate to have nginx installed on the system for Setting up offline page but that requires understanding how ports work under Linux. 
So don’t start there if the current situation doesn’t make sense to you!)