On my Lumia925 with Windows 10 and on Windows 10 Desktop it works as iCloud Account and the URL’s https://subdomain.yourdomain.com/remote.php/dav/ for all the Calendars and https://subdomain.yourdomain.com/remote.php/dav/addressbooks/users/<username>/contacts/
If the above URL’s didn’t work for you then either you have a Problem with your Phone or the something is wrong at your Server. I didn’t use my Windows Phone any more after giving up my Work on my KeePass Client for WP. But that’s a other Story. But you can use the Windows 10 on a PC and the Calendar App there, with the same URL’s to test your Calendar’s. If something goes wrong, you should see some entry’s in the Server Logfiles of your Webserver and/or Nextcloud Logfiles.
I have Hardreset my Lumia925 and entered the above URL’s and Calendar and Contacts appear in the normal Apps. I have access to all Events in the Calendars.
BTW: Check if the Server does have a official and valid SSL Certificate. If this is not the case, Windows Phone declines communication with the Server other https. Sure, you can import such a Certificate manually to the trusted Store on Windows Phone, but i wouldn’t recommend that.
Maybe remote.php/caldav/ /remote.php/caldav/principals/user/ (OSX)
→ I know it’s a little bit weird, but that was the old link and worked for me (macOS).