

OK you are thinking at it the wrong way : 80 and 443 are the default ports for the web access of any web application.
You are using docker with the nextcloud aio image. Which mean, you are deploying a docker container which contain the web application.
What you are doing is using arbitrary ports for the container. This could be 7777. Then, this is redirected to 443 inside the container, to the web application.
This is why you can actually use any port you want for your container. You are not really switching the port for nextcloud, only for the container.
This is why I suggested doing 444:443 in the docker-compose.yml file: you are exposing 444 to the container, to 443 inside the container.
And this is why you can actually use any ports for every web app you are going to host : apache will redirect to the port you want to the container, a’d docker will redirect inside the container to either 80 or 443 (or anything else needed for that web application).
You could probably go for nextcloud.yourdomain.ddns.org, but I can’t guarantee that since I have no experience with ddns.org.
There is a lot of cheap registrars, I’ll let you check and select the one you want if you need a second domain.
Basically docker let you setup multiple (fake) computers inside your computer. For those that want to correct this, I know. But I’m trying to explain the top view here.
So, your computer (host) is hosting multiple containers, each containers will have there set of available ports. And as stated earlier each container will have an application that may or may not need to expose ports. But since each have their own ports you can have hundreds of containers using the port 80 INSIDE the container, an using 8100 to 8200 OUTSIDE the container (on your host). The only real limit would be the amount of ports available (somewhere along the lines of 65535).
There. Now go have fun and read up on docker and reverse proxy! Don’t forget to use ssl on your nextcloud instance since it’ll be facing the internet! And… Have fun! :-)