I understand you have changed TV providers, and need to reconfigure the video input. I see you have the DirecTV C51-100 model. The remote code that will work with this is the C41 model. Here is a handy tool that will help you find your set top box model.
I hope this helps!
Sling Media Support
I just switched to DirecTV from AT&T Uverse and am having trouble connecting my M1 Sling Box. I keep getting a no Video Signal error message. I tried to reconfigure the Video Input, but cannot find my exact DirecTV set top box model (C51-100). Can anybody help me?