You can set up your Calendar sync automatically through your webmail. Tap on the gear icon on the top right, then tap Connect your device and follow the steps. 

You can also set it up manually

1. Tap Settings.

2. Select Mail, Contacts, Calendars.

3. Tap Add Account.

4. Select Other.

5. Under Calendars tap Add CalDAV Account.

6. Enter the following details for your account:
   • Server: dav.rebel.com
   • User Name: The email address that the calendar is associated with.
   • Password: The password you use to log into Webmail.
   • Description: Give it a name so it is easy to recognise.
7. Tap Next in the upper right corner to verify the account.
8. Tap Save in the top right corner.