FortiNAC is extremely capricious software, any little parameter (from hundreds) in your env that is not configured as it wants will lead to various kinds of issues. I can tell you that FortiNAC is the most capricious and complicated software I've seen in my 15 years career.
So for your first issue, the portal may not auto-display until you request some URL on your browser, or may be caused from DHCP address is coming probably late because your client may have not refreshed its dynamic address when it should do, this is one or two of the many possible scenarios. So try check your client's IP address while you are waiting for the portal to display.
For the second issue, as far as I remember it happens when you don't have valid certificate for on your server portal, or if the certificate is not recognized by client, in version 9 dissoluble agent is enforced with https. So when you are prompted you should enter protocol before IP, i.e.: http://x.x.x.x/some_path, this should work.
I think there is probably "some_path" but sorry I don't remember it.