How to setup NextCloud on Linux while running a VPN?

Yes exactly i followed the All In One installation (AIO) guide from the Github for Linux and managed to successfully install the Docker container and the next cloud AIO
Then I:
1.Opened my web browser
2.Entered https://localhost:8080
3.Accessed the page i added in the post
4.My ip is dynamic for 2 reasons:
1.It’s a residential internet and by default it always change
2.I am always using a vpn for my privacy so again that means my IP can change constantly
So to overcome the need for a fixed IP address that is needed in that page
I went to the alternative solution which is to create and use a Dynamic DNS (DDNS)
I created one using this service: deSEC
5.I entered that created domain in the requested field as in the picture here:

6.Then i clicked Submit Domain
7.Finally i get this error:
”Domain does not point to this server or the reverse proxy is not configured correctly. See the mastercontainer logs for more details. (‘sudo docker logs -f nextcloud-aio-mastercontainer’) If you should be using Cloudflare, make sure to disable the Cloudflare Proxy feature as it might block the domain validation. Same for any other firewall or service that blocks unencrypted access on port 443.”
8.I tried to fix it by opening the firewall app in linux and setting an exception for the mentioned ports 443, 80 and 8080
But that doesn’t make any differnce
Then i tried to turn off the VPN
But again this didn’t make any difference

And finally to exactly answer your question:

The main machine
The same machine
All of what i mentioned here is happening on the same computer the one i am using for my daily use that has vpn on it and has the docker with the next cloud intaller on it and the one i am trying to set the nextcloud on as the “Server” or the “Cloud”
And regarding port forwarding
The VPN has port forwarding but it doesn’t allow me to select which port to open
So now you have all the details
(If you need anything else please ask)
What should i do now ?
Where do i go from here ?
I know it’s not ideal but let’s say i want to create a system like this
How would that be possible ?
There has to be a way
P.S
I forget to mention that i installed the DDNS or connected the domain i created with my system using an app for linux called ddclient in the Debian repo.

Excuse me if some of my terms are not as accurate as possible but i am new to self hosting and doing my best to learn