Certificate for FortiNAC Portal

Hi all,

We want to set up a captive portal for guest users, on our FortiNAC.  We're currenty using a selfsigned certificate, issued by our internal CA.  When we connect to this portal using domain computers, this seems to work fine because the CA is trusted.  But when external consultants want to connect to the portal, they're getting an SSL error in their browser because the certificate isn't trusted.

We do have a wildcard certificate for our external (public) hostname, issued by an external CA, but I suppose we can't use this certificate for our portal?

Thanks for feedback!