calendar issues

Try going to iCloud.com using a computer to see if the items are there and whether they are from the device or the computer. That should show whether the device or the computer isn't syncing. If the computer, try going to System Preferences/iCloud and stop syncing. Wait a couple of minutes, then recheck syncing. If the device, go to Settings/Your Name/iCloud and do the above. As a last resort, you can also try signing out of iCloud and then sign in again.

Contacts, Calendars, or Reminders - Get help using iCloud

The iOS device should be updated and upgraded see these articles How to update to iOS 12 - Official Apple Support

https://secure2.store.apple.com/shop/eligibility/upgradeEligibilitySignInOptions ?_np=standard/home/shop_iphone/family/ip…

How to Upgrade Your iPhone - Apple

Also your Mac and iOS must be signed in with same Apple ID and password having essential network speed to sync calendar and its events that you create in any desired machine .

See this article Keep your Calendar up to date with iCloud - Apple Support

You are welcome.