Yes , I don't think self-certified instances are supported. You can use http though obviously it's not a good idea in general 😄

@libremax

Nextcloud has a lot of API open to developped apps, the main problem currently is a lack of developer on our side.

For me it just worked "out of the box" on my phone and tablet. My config :
Account Settings
Username & Password (same format that when you log in your server website, for me it is just the username, not username@domain.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" !)

Sync Folders
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

Sync service
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 tried too with Unixcorn : https://unixcorn.org/
with https://cloud.unixcorn.org and a 12.0.2 Nextcloud version but it never worked for me (no sync).

I suppose that Owncloud-Sync doesn't works with Nextcloud. It would be great thought !
I hope all of this could help you.