Cant access userman from DNS CNAME record

My upstream ISP does not do static public IP addresses so I have had to fudge my way through this. Basically, I have a RB Crossroads board setup as a hotspot and want my users on the wireless network to goto http://RouterIP/userman and create an account. This way works fine however but when you replace the RouterIP with the domain name it shows a 404 not found error.

I am using DDNS to point a CNAME record to the correct FQDN and the status page comes up fine on hotspot.domain.com/status but pops the 404 error on hotspot.domain.com/userman

any ideas on how to fix this?