Assuming that you have updated your iPhone11 to iOS13/14, when opening your reminders App for the first time, you were presented with an option to “upgrade” the Reminders App. You will also have been presented with a warning that compatibility with devices running earlier versions of iOS would be lost.
If you clicked-though and accepted the upgrade, you triggered a one-way update for your account on Apple’s servers. Once “upgraded”, your iPad with an earlier version of iOS ceased to be compatible with your iPhone/iCloud Reminders - and this may be the source of your problem.
Upgrading the Reminders app in iOS 13 or later - Apple Support