In order to sync, you must first be on Premium Service on both devices.
First, you need to charge for the premium service on one device and use iCloud backup on the other. You can do this. On the other device, under the billing button on the Premium Services screen, you can see the "Restore" button, you can become a premium plan without paying for it. The following is a brief description of the process.
As for synchronization, since it is not automatic synchronization. At first, please tap the "Create a backup". Then, on the device of the person who wants to take over the data, do the "Restore iCloud backup". The data will now be synchronized.
In case you are having trouble, please check the following.
* Open the "Settings" app and tap the Apple ID field at the top, then select "iCloud". Tap to make sure you have plenty of free storage space in iCloud.
* Please make sure you have the latest version of the app on both devices.
* After deleting the iCloud backup once, and creating it again, try "Restore" please
* Please reboot your device and try again.
If you've done all this and it still doesn't work, you can use the "Contact Us" button in the upper right corner to find out where you can get stuck. I would appreciate it if you could tell me about it.