12.4 Update Calendar Issues

What was your previous iOS version, that is important, because iOS 12.4 only had the following changes. Please introspect how these changes can affect what you have posted.

Phone migration

• Introduces the ability to wirelessly transfer data and migrate directly from an old iPhone to a new iPhone during setup

Apple News

• Makes downloaded issues accessible in the My Magazines section, both online and offline
• Adds all publications in Apple News+, including newspapers, to the catalog at the top of the News+ feed
• Adds the ability to clear downloaded magazine issues by selecting History > Clear > Clear All

Other improvements and fixes

• Includes a security fix for the Walkie-Talkie app on Apple Watch and re-enables Walkie-Talkie functionality

This release also includes support for HomePod in Japan and Taiwan.

Nope that hasn't helped.

The online iCloud is correct, our iMac Calendar similarly so but since 12.4 update iOS devices not syncing. IOS have been updated every time there has been an update. Have deleted accounts, deleted apps etc but still not working properly

The easiest way to fix that problem would be, in my opinion, to delete all of the accounts and add them back.