How do I recover calendar from iCloud to my iPhone?

See the article Set up mail, contacts, and calendar accounts - Apple Support

Set up a calendar account

1. Go to Settings  > Calendar > Accounts > Add Account.
2. Tap Other, then do any of the following:

• • Add a calendar account: Tap Add CalDAV Account, then enter your server and account information; see Set up multiple calendars on iPhone.
  • Subscribe to iCal (.ics) calendars: Tap Add Subscribed Calendar, then enter the URL of the .ics file to subscribe to; or import an .ics file from Mail.