I fixed it a different way.
1. SSH into your FreeNAS box and become root.
2. invade the root of the jail: # cd /mnt/zones/iocage/jails/nextcloud/root/
3. find config.php: # find . -name "config.php"
4. edit config.php: # vi usr/local/www/nextcloud/config/config.php
5. Find the trusted_domains block and add the IP address of the Jail: (in my case, since the URL offered by "Manage" in the FreeNAS Plugins UI was offered as
https://192.168.2.207/login, use that IP
6. In the Plugins UI panel, Stop and Start the Jail
7. Hit that "Manage" Button - a new browser window will launch
8. Get the password for user 'ncadmin' from the "Post Install Notes" Icon offered in the Plugins UI panel.