How to register old version of Cubase AI

I decided to return to my old version on Cubase AI 5 attached to CC121 I bought over 10 years ago. It has never been installed or registered before. When I tried to do it from the option “Register now” it links me to the page that doesn’t exists (service. steinberg. de/goto.nsf/show/register_cubase_ai_5_gb).
How to register this product for permanent use?
Thank you for help.

You can register it here:

It looks strange because I’ve opened envelope but there is no activation code…, but in manual is info that this version must be registered to work longer than 30 days. Is there any other solution?