Manually
(this one always works)
- Tap the Settings icon on your home screen
- In iOS11 go to Accounts & Passwords
(or Calendar in iOS10 or Mail, Contacts, Calendars in iOS9 and below) - Tap Subscribed Calendars.
- Select the relevant subscription
- Copy the Server field URL
- Paste the URL in an email and send it to your other devices
- Open the email on the other device(s) and tap the link to add the subscription
OSX
See paragraph Manually above, clicking URL will add it to your calendars.
Using iCloud
All your in-app purchases will be synced between devices if you have iCloud enabled (required) with the following settings:
- System settings > iCloud > Calendars ON
- System settings > iCloud > iCloud drive ON (or Documents and data ON if you did not turn on iCloud drive)
- Give app access here
The subscriptions should then be visible.
If you can't see your purchases yet, please reboot your device, since it might take some time for the iCloud service to sync your subscriptions.