I have no idea what I'm doing

I installed WordPress on my server here at work that’s running Windows Server 2008 R2.

We have a domain name that we purchased from domainsite.com several years ago before I worked here. I have the login to manage the account, but I have no idea how it’s set up. We do not use them to host our site.

We had our old website running on our old server with Windows Server 2003, and I wanted to leave that alone while building the new site. I built the new site using the static IP from the new server so I could access it anywhere on our intranet.

When I was done building the site, I shut down the old server and assigned the new server the old one’s IP. I went through the new wordpress site and updated everything so it still works on the intranet.

Now if I try to access our url from outside the intranet, I get the text from the page, but no formatting or images. I know this is because all the links are set to the local intranet IP address.

If I try to change the “WordPress Address (URL)” to my https://www.oursite.com URL, I get locked out and can’t access anything and the whole site goes down.

My question is, how do I proceed from here in getting everything changed over to be viewed outside my intranet?

I have no idea how to use myPHP or any of that stuff. The most complex thing I’ve done is edit something in notepad, so I need extremely basic instructions. I’ve found a ton of other threads but they are all way over my head. Can anyone help me?