sync iphone ipad calendar

Calendars can be associated with email accounts. So you sync calendars through iCloud and/or email accounts. If there are email accounts that are connected to one of your devices (for calendar purposes), and that have not been connected to the other device (for calendar purposes), that may explain lack of synchronization.

Set up mail, contacts, and calendar accounts on iPad - Apple Support

If you have "looked at everything on the web and forums", there is likely very little that contributors here will be able to add; without explicit information as to what you might have already found, we can only begin to guess.

So, starting at the beginning, ensure that both your iPad and iPhone are signed-in to iCloud with the same Apple Account (formerly known as your AppleID). Next, verify that you have enabled iCloud Calendar on both devices:

Settings > [Your Name / Apple Account] > iCloud > [Saved to iCloud] See All > iCloud Calendar > Enable on this iPad | iPhone - set to ON

You need to make sure you are using an iCloud calendar on both, and not local calendars: Set up multiple calendars on iPad - Apple Support