ios 11.1.1

You must have been using iCloud for your calendar or it would not have been there. Glad you were able to recover your data, but understand at this point that you are managing your calendar with iCloud. And if you go to Settings>Your Name>iCloud, you will see the switches for things you are managing with iCloud.

How were you managing your calendar, via iCloud? If so, log into iCloud on a computer and see if all of the information is there. You can then check on the iPhone to see if Calendar is turned on in Settings>iCloud. You can try power cycling the phone and see if that triggers the sync again. There are other steps, but let's see if this works.

You're welcome. Take care.