I have looked over the intstalltion PDF for the 303H but really doesn't help much, but the 2nd link you gave does make sense because the internal network does not have a DNS not found page, but my home network, and our lab network does have that page. My home network ill try to block that DNS failure page with my firewall it seems to work.
If I provision my RAP on the internal network using the internal controller IP will the RAP work from the internet how does it know what the external IP address is after it's provisioned. Does it only get that from when you convert the rap from IAP to RAP?