This is more about networking and routing than Confluence, but it's always a good question.
You need to establish a route from <external> to where-ever your Confluence is running. The simple answer here is "talk to your network administrators" - it's their job really, as they need you to tell them where it should be located and then they can configure the way to do it.
As a rough example, I am rebuilding an Atlassian stack at home. Confluence, JIRA, etc. The servers running them work fine when I hit 192.168.0.120/121/122/..., when I'm at home. To make them accessible when my laptop is not at home, I told my router to forward incoming https traffic to a web-server that proxies them (so https://me/jira goes to 192.168.0.120, https://me/conf goes to 192.168.0.1 and so-on)
That really is a very very simple case, but hopefully, it gives you a starting point to take to your network team. They need to give your external users a route to your Confluence.
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