You used wrong guide/article, it is long ago outdated and shall be avoided.
For IIS stuff, I no longer recommend to use IIS Manager to generate requests, because its functionality is limited and outdated. Instead, you should use Certificates MMC snap-in, for example as outlined in my weblog: Web server certificate enrollment with SAN extension. One thing you should consider -- you MUST specify Subject Alternative Names extension, otherwise neither of modern browsers would trust your certificates. And forget about any article that mentions enrollment web pages, it is dead and any article that mentions them is dead too.
NPS/Radius is another beast and uses different approach, which is much simpler: you can use certificate autoenrollment to automatically request and renew NPS certificates.