Opening your ports:

The procedure to open your ports is different for every router. The best place to go would be [Only admins are allowed to see this link]. Just search your router and follow the BASIC (not WoW) guide on opening the following ports:

• 80 (Default port for Apache servers. If you aren’t running a site of sorts, this is not required.)
  
• 3306 (Default port for MySQL servers. If you don’t need your MySQL to be accessible remotely, this is not required.)
  
• 3443 (Default port for Remote Access, if you aren’t using RA, this is not required.)
  
• 7878 (Default port for Simple Object Access Protocol, if you aren’t using SOAP, this is not required.)
  
• 3724 (Mandatory realmlist port.)
  
• 8085 (Mandatory World server port.)
  

Once done please check if your ports are open using [Only admins are allowed to see this link] site. Don’t use other tools, as they’ll probably be incorrect.

Getting a DNS:

A DNS will serve as your realmlist, for example Blackrock-Gaming uses ‘realm.blackrock-gaming.com‘ of course, you’d need a domain name for that; but there are free sites to do that, such as [Only admins are allowed to see this link].
[Only admins are allowed to see this image]
Your ‘IP’ address is the IP address that you can find [Only admins are allowed to see this link]. Make sure the setting is ‘Host with IP address’ and nothing else.

Changing the HOSTS file:

This step is important, since you are the HOST of the server, the
computer should be expecting incoming connections from an IP. By
default, the computer only expects incoming connections from ‘127.0.0.1‘, that is your local host. Navigate to ‘C:/Windows/System32/Drivers/etc/‘ and open the ‘hosts’ file with notepad and make sure you edit it:

# localhost name resolution is handled within DNS itself.
# 127.0.0.1 localhost
# [Only admins are allowed to see this link] mydyndns

Once done, just close and safe.

Changing the realmlist:
This last step is forgotten by many and is 'the most important' step to actually get your-
Trinity Core to be public. Open your database via HeidiSQL, go into the 'auth' database and -
open the 'realmlist' table. There, change the address from '127.0.0.1' to '[Only admins are allowed to see this link]'
Close the table. Success! You're done!.

Outro:
If you did the above correctly your server should be public. If not, it could be that your
firewall or anti-virus is still blocking connections, if so, just disable them.

Players can reach your server by setting their realmlist to either your external IP address or your DYNDNS.
'Protecting' your IP address is useless, as anyone with a brain can simply ping the DYNDNS and figure out what your IP is.
A hacker can't do anything with just your IP either, so stop being paranoid.

Please by advised that you SHOULD NOT change anything within the actual config files of Trinity, these settings are correct by default. Do not change them unless you know what you're doing.