Subdomain question

Hello I`m trying to setup a subdomain behind the untangle.
I'm at 9.4.2.
I`ve a Domain (whatever.com) is hosted domain. Under that domain is hosted my website. On my hosting DNS i´ve create a subdomain and point ut.whatever.com to my untangle`s external IP. My hosting give me a little client that allows updaten external IP dinamically.
In my untangle i`ve setup my Untangle hostname as UT.whatever.com and Domain sufix: whatever.com
.
Now i want to create a new subdomain rd.ut.whatever.com and redirect the trafic to one of my internal servers.
At this point i`m lost. I don`t know how can i do. How can i create the new subdomain as rd.UT.whatever.com?
can somebody help me?

Thanks
Edu

P.S. sorry for my bad english i`m from spain.