Diarly uses iCloud to store all the information regarding your purchases. This allows user to pay once and use the app on Mac, iPhone, iPad under one subscription.
To purchase a Diarly subscription, please :
Make sure you are logged with the same iCloud account on all devices you want to use the premium version of Diarly.
Make sure you have iCloud Drive enabled and Diarly has access to it on all devices:
Please follow these instructions: https://support.apple.com/en-us/HT204025 to make sure you have your iCloud Drive set up correctly.
Once you purchase subscription, all other devices using your iCloud account will sync the data (if you enabled iCloud sync) and allow you to use the premium version.