I paid for a Premium license for multiple devices, but I do not see where to add the Activation Code on the iOS version of CCleaner.
CCleaner for iOS is fairly new and doesn't seem to have made it into the help documentation yet.
Apple had resisted one for iOS (although the MacOS version has been around for a while), and I have no idea what licencing agreement/arrangement was put in place to now allow it.
The best I can suggest is that you reach out to support and ask them.
You can raise a support ticket by using the form here: https://support.ccleaner.com/s/contact-form?language=en_US&form=general
There is an option for iOS in the 'Platform' box, so at least that has caught up.
You can also contact CCleaner support by emailing: support@ccleaner.com
(If you have bought CCleaner Premium then you also have access to 24/7 live chat, see your purchase receipt for the link).