I have purchased freedome VPN with three licenses and I do not know how to activate one of them for my iphone.
Maybe this Knowledgebase/How-to articles should be helpful:
Does it work for you?
If not - does there any visible prompts/notifications about trouble? Such as - if license not accepted; or just if there trouble with installing Freedome? About some potential troubles with vouchers can be this article:
Also based on F-Secure website -> F-Secure Freedome support 'iOS 9 or later';
Sorry for my reply.