Hi everyone!!
We're experiencing something similar, we have a microserver that is DHCP and DNS server, the captive portal internal and external works fine conected to other LAN with Internet and Google DNS, but when the AP is connected to the MicroServer without Internet the captive portal not only doesn't pop up, but also doesn't work if you open a browser and write the domain name (google.com, arubanetworks.com, etc) it sends the "cannot find server" message, but if we type some ip address (1.1.1.1, 10.0.2.3, etc) then redirects to portal captive (only if internal), when the external CP enable it sends a "404 page not found" message.
The reason for this config without internet is that we have to move the server to several locations to conduct a survey using tablets that would connect to a small database also hosted on the server.
So we are kind of stuck there.
Greetings!!