Reminders and Contacts disappeared when turning on iCloud
On MacOS, each of the 4 apps you refer to support both local and iCloud-synced accounts or lists. 

To avoid any syncing issues and for troubleshooting, I suggest the following:


To start, leaving iCloud off for each app on each device.
Then turn on iCloud sync for the first one, e.g. macOS Contacts.
Pay attention to what happens. You may now see both an 'on my mac' and a new 'icloud' group appear in the left hand list.
If there are no contacts in the 'cloud' section, then perhaps they are only inside the local copy in the 'on my mac' group. 
If so, then move them into the 'cloud' group. Does it look like there is a network operations (spinner) doing stuff in the corner? If so, then things are going well. 
Wait a while until everything is done. 
Now turn it on in the other advice and again wait for things to sync. Also check if there are contacts 'on this iPhone' that you also want in the cloud.
Same concept applies to the other 3 apps, with minor technical differences.


Hope this helps!


Damon, aka Semiconducted