Brian,
I assume that you just grabbed the .cer file and imported it into the IIS mailbox. Unfortunately, this is not possible, since your certificate will not contain a private key, so it will not be an option in IIS.
The GlobalSign team you specified is great, but if you created the CSR on the IIS server itself.
What you need to do is create PFX from your .key and .cer files from the Apache server, and then you can import this PFX into your IIS server and designate it for use.
So use this to create your PFX file; https://support.globalsign.com/customer/portal/articles/1229676-back-up-certificate---apache-openssl
Following; https://support.globalsign.com/customer/portal/articles/1217281
As for everyone else, if you only have a .CER file, you have not created a request, you need a secret key. So create a new CSR and ask for a reissue from your respective certification authority.
The fact that you cannot assign a certificate in IIS is that you do not have a private key that matches the certificate.
Rickk source share