UHF and RF are in this case conceptually the same thing. In terms of technology they are a bit different, but it's the same concept for controlling your sat box through a wall.

Now, I believe that 921 takes both IR and UHF at the same time. So it shouldn't be an issue using a universal remote to operate it via IR.

Those remotes do have some RF options, but these cannot directly control your 921 via RF, it is not compatible. If you get the RF option, you will get a little RF receiver that will output IR to the 921. No universal remote can directly operate a device that takes RF/UHF from its own remote.

Check this out: [Link: remotecentral.com]