I assume you have already done a proper Forum Search for your model number in case other people have already answered your question ?
If so and that didn't help then check out this thread: https://community.sling.com/message/10363#10363
Come back if it that doesn't help either.
There are a couple of different ways you can go about that; I would recommend trying the "U-Verse Black" remote. If this doesn't work, I would ensure that the set top box is on IR and not RF and that, if you are using it, the IR Blaster is directly infront of the IR Sensor in the set top box.
If these steps do not prove fruitful; it is possible we do not have the coding for that remote just yet. You can search the forums to see if that remote has already been requested or created and work with the bin file from there (or make a request if need be).
I hope this helps!!