As the Tuner in this box is analog I assume you are driving it from the output of the DTA box ? This analog output will ONLY be Channel 3 or Channel 4, which would explain why only one channel was found. So that is normal behavior.
Forget about the provider, that is simply to give you an on-screen TV Guide, it doesn't relate to the operation of the Slingbox.
And surprised your Comcast RNG100 doesn't show up as that is a pretty standard box. I would try some other Comcast boxes as they all tend to use the same codes.