I have Nextcloud 19.0.5 running on Freenas.
I’ve installed and enabled the two apps: Collabora Online (3.7.9) and Built-in CODE Server (6.4.14 ).
In the collabora online settings I’ve only two options
use your own server and
use a demo server.
Looking around it look like I should have a third option to connect to my Built-in server.
Uploading the screenshots