Can't get access to my Nextcloud outside of my network

I’ll start by saying that I’m very new to this.
I managed to install Nextcloud by watching a lot of videos on YouTube, but I had never worked on linux or anything to do with servers or networks.

My Nextcloud is working perfectly after much frustration.
I managed to install the domain, and do the port forwarding, I enabled the ports in my firewall, but still when I try to access my Nexcloud from another network I can’t.

I’ve done everything the tutorials I’ve found have told me and nothing works for me.

Any idea what I might be missing, or someone willing to help me out?

What kind of Network do you have?

Do you have a registred Internet Adress (dynamic DNS) or can you build a VPN connection to your home router?

Your home network is most likely behind a NAT firewall. So you need to know the internet address of your home network. If this changes regularly (depending on your provider), then you need a dyn-DNS service, which ideally gets an update from your router every time your Internet address changes. If you have a fixed IP address, then you can view your network at that IP address.

Another option is to establish a VPN connection with your router (assuming it supports it) and then you can access your server again using the internal network address.

Yes I have a dynamic DNS.
How can I do the VPN?

You only give very rudimentary information that is not sufficient for my crystal ball to see a solution. Sometimes working can be just a semicolon away from not working. You have to give a little more insights and, of course, be willing to learn a lot about how everything works and sticks together.

This is a fundamental question that actually has nothing to do with Nextcloud so far. This is part of the digital general knowledge.
Without coming across as unfriendly or dismissive, would I like to encourage you, to first familiarize yourself with such basics and continue to develop your skills. Use tools like Wikipedia to get an overview, search engines and/or ChatGPT for fundamental questions like how you have to set up a vpn that fits your needs.
The web is full of guides on setting up a VPN.

In short, either your dynamic DNS is not working, or your port forward is not working. I would verify the DDNS first since that’s trivial to check, and then move on to troubleshooting the port forward.