If you don’t manage to get the built-in AIO reverse proxy working the way you want, you can also run Nextcloud AIO behind your own reverse proxy — for example NGINX Proxy Manager.
That’s often more flexible long-term, because you can use the same proxy for your other services too, not only Nextcloud.
I wrote a full step-by-step guide for Proxmox → NGINX Proxy Manager → Nextcloud AIO, including domain setup and port forwarding:
Proxmox + NGINX + Nextcloud AIO + Watchtower Deployment Guide
It’s an alternative approach that might fit better if you plan to host multiple services behind one proxy.