Since you've added the alias it should of added it for you, but you could check the vhost config for that site and compare it with the tutorial you're following:
enhance - nginx vhost conf
you'll need to ensure your alias is correctly listed as a server_name and then re-save your permlinik settings for wordpress.
Give that a go... else there are some people on here that use nginx and can hopefully direct you better I am more into litespeed web servers.