Captive Portal issue

Hi,

I have the following issue.

When we connect to Guest SSID, Captive portal page not coming.
So, Do we need external DNS server to resolve ov2500-upam-cportal.al-enterprise.com to IP?
If so DNS A record to be configured in DNS to resolve ov2500-upam-cportal.al-enterprise.com should be OV2500 IP or Captive portal IP?

Regards,
Riyas