Xfinity Boxes can work as RF radio frequency or IR infrared. Sometimes the installer did not "activate IR" in the box. Flipper, like all universal remotes, is IR infrared.
So lets make sure IR is activated in the box.
Using Xfinity Remote:
Menu-Settings-Remote Control-Activate IR.
Or something similar. You may need to check with Xfinity or look online for your specific model box for the exact instructions.
Once we make sure IR is activated in the box, then Xfinity is always code 3084.