Accessing Sharepoint using Ip address and port

Hi,

So I've succesfuly set up a Sharepoint 2013 Foundation using SQL Express 2014.

I can access the Central administration site from inside the network AND outside the network without any problems.

However, for the User website I have a strange thing going. 

Before you read any further, you need to know that I'm not responsible for the security part of the network.

I've asked the network administrator to open up ports so I can access the website (admin and user website) from outside the network.
The guy responsible for this did this. However he assigned one port for the administration site (So it's the same for when you want to access it from inside the network and outside).
BUT he assigned 2 ports for the user website. One for inside the network (standard 80), and for when you're outside the network (another port)

When I'm trying to access the user website from outside the network I have to specify Ip address and port. This works but sometimes he does a redirect and then he loses the port and therefore he can't find the page.

So things that occure are:

- redirecting to URL without port --> result: page not found

- when specifying url explicit, he finds page but some parts are not loading

- when I do I have a page that works, and I do a certain action and sharepoint redirects me, he sometimes uses the port and sometimes not which results in Page Not Found situations.

My question:

What's the cause of this? Is it because I use a different port for inside the network and outside the network?
Could this be solved by specifying a single port for inside the network and outside the network? Or is there something else going on?

Would it be as simple as asking the network administrator for opening a new port and then creating a new web application using the port the network administrator opened?

Kind regards