Apple is really good at making things simple. AirPort Extreme is no exception. I will be showing items in the Apple Time Capsule. For the price, the Time Capsule is $299 where the AirPort Extreme is $179. For what you are getting both are very acceptable.
Let me start by saying I have had my TimeCapsule since about 2008. It has had no problems and has been a backup device for my two laptops since I got the TimeCapsule. I would personally recommend the AirPort Extreme since the network backup is nice to have but you can go a much cheaper route by getting a larger network storage device.
3) Next assign the port for connecting from the outside world (e.g. Public UDP/TCP Ports)
For security reasons, you should pick a number above 40000. This is so port scanners can not detect what service might be running on your IP Address. This is important to prevent hackers from getting into your network or using the item you are port fowarding.
I will be port-forwarding plexapp (see my technology section for more posts on plex app)
Now that you have your port fowarding. You are able to access that port externally. One way to test this is get your external ip address (you can use the script I posted a little while back or use google: external ip address). You should be able to setup your PlexApp device (iPad/iPhone) by adding a custom server, entering your external ip address and port number. If you did everything correctly you should be able to connect no problem.
If you can not connect. Make sure you have no hardware/software firewalls preventing your connection. The computer you setup the port fowarding for needs to also be able to talk to the AirPort Extreme. Sometimes you need to “allow” the port access so your Airport can foward the port outside. If you are using Little Snitch it will ask you the first time you try to connect to the port (internal or external).
As always, be conserned about security. Plex app for example you can enable security which forces a username/password for anyone who trys to connect to that port.
You can also apply the same concepts to port fowarding a VNC connection. Of course you need to use a different external port for each device you want to access outside of your home. I recommend only port forwarding what is absolutely necessary. Less is more in the case. You don’t want 20 ports all exposed to the outside world.