These simple steps show how to add an CalDAV task lists.
Install OneTask
Download OneTask from the store and install it on your device.
Go to OneTask Menu (top left)
Select accounts overview
Select Add account
Select account type
Important: for Nextcloud, Synology and Mailbox.org are dedicated accounttypes you should then use one of these. For all others just use CalDAV account type
Enter credentials
Now fill in Username, Password and ServerUrl. For server url you can just enter the hostname e.g. "test.domain.com" without any additional info, OneTask will try to auto detect the correct URL. If that does not work please lookup the correct URL from your CalDAV provider or ask your CalDAV provider. The URL can then start with "https://"