ICloud – Reminders and Contacts disappeared when turning on iCloud

contacts.appicloudreminders

I decided to give iCloud a go, so I did the following:

The following happened:

Calendar events were merged (that is, they were copied from iPhone to Mac, because I had no calendar events on Mac). So that was OK.
My notes were left on each device, because judging from the GUI you manually control if they are local or iCloud. So that's OK too.
Contacts were merged on Mac. All contacts disappeared on iPhone (they returned when I turned off "Contacts" in iCloud settings). Not good.
All my reminders disappeared from Mac, so I panicked and turned off iCloud on Mac, and then back on. Some of the reminders appeared on both devices, but many were missing. Disaster!

I fear to touch anything now, afraid that I'll lose more data. My questions:

How do I get contacts to show on iPhone (with "iPhone" turned on in iCloud settings?)
Are the reminders on Mac lost, or how can I get them back? (I don't use Time Machine.)

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!