Excuse me, I don’t understand your question.
And I fear I won’t be able to answer it without further knowledge about your setup, what you proxy how and to whom and how you setup and tested nextcloud so far.
You should really look into the installation guide of nextcloud
https://docs.nextcloud.com/server/13/admin_manual/installation/nginx.html
setup nextcloud as you desire listening on the port of your desire, which means: just modify the ports as you need them in the example config and check if you can access nextcloud on that port and if everything is working fine.
Let’s say: yourdomain.com/nextcloud:8443
That should work first. If it does, than you can start to setup the proxy accordingly. It will be easier then to debug issues. Doing all at once will only lead to confusing, because you don’t know if the configuration of Nextcloud is incorrect or the proxy settings.