For me it just worked "out of the box" on my phone and tablet. My config :
Username & Password (same format that when you log in your server website, for me it is just the username, not firstname.lastname@example.org)
Server URL : https://webcloud.zaclys.com
Sync on mobile data checked (but it also works unchecked)
Sync frequency : 60 minutes (what you want but don't choose "No sync" !)
Local Folder : /home/phablet/Documents
Remote Folder : /Documents
Local Folder : /home/phablet/Pictures/com.ubuntu.camera
Remote Folder : /Photos/Aquaris
Local Folder : /home/phablet/Videos/com.ubuntu.camera
Remote Folder : /Videos/Aquaris
you have to activate the service
you can manually sync to test your config
I use a server from Zaclys : https://www.zaclys.com/
They use the 8.0.4 Owncloud version but they offer the Nextcloud 12 version too.
I suppose that Owncloud-Sync doesn't works with Nextcloud. It would be great thought !
I hope all of this could help you.