Go to Settings/[your name]/iCloud and turn on Photos, then wait for them to sync.
Have you verified that they are on https://icloud.com/photos? Note that backing up an iPhone to iCloud is not the same as syncing an iPhone to iCloud.
iCloud+ has 2 independent functions. iCloud backups, and iCloud sync. It’s confusing because they both have the same name prefix. But they have no connection with each other. Go to Settings/[your name]/iCloud. You will see a bunch of switches. When you turn on a switch that data type will sync to iCloud. If you do this on multiple devices that share an Apple ID the selected data will sync to all of those devices.Â
You can also turn on iCloud Backups. This will back up your phone every night if the phone is plugged in, connected to Wi-Fi and locked.
But note that anything that you sync by the first method will be excluded from the iCloud backups.