What I personally think you should do is ditch the plugin and its messy use of NAT (which, AFAICT, makes it impossible to implement TLS there), and use my script instead:
Status: This script will run with TrueNAS CORE 13. It's possible, but untested, that it will work with FreeNAS 12. Earlier versions are not supported. There are a number of guides on the forum to install Nextcloud, but they all rely on a lot of...
www.truenas.com
Then Nextcloud will have its own IP address, distinct from the FreeNAS box. Forward ports 80 and 443 to that IP address. As a bonus, it will handle TLS certificates for you too.
If you don't want to do that (perhaps because you already have data in the Nextcloud plugin), set up a reverse proxy that will handle TLS termination for you. I have a guide for that as well:
Many users install a variety of web applications in jails on their FreeNAS servers, and often those applications run on non-standard ports like 6789, 8181, 7878, etc. These port numbers are far from intuitive, and the applications often either...
www.truenas.com
Then forward 80/443 to that jail's IP address. If you'd rather use a different reverse proxy software instead, there are other guides in the Resources section using Nginx and perhaps other software as well.