iPhone app remote control

Hello
I’ve done a little bit of research and it seems like the apps available on an iPhone are similar to a universal remote.  I’ve downloaded an app for my iPhone and it’s connecting with my TV. I can change the volume. The only thing I cannot get it to work is to change the channel when I press Guide on the app. It does not bring up the Cox channel listing..  

Could anyone offer some guidance if there is a setting or something I need to do to make the app channel light up work on my iPhone app please?

Thank you very much