On the cox remote the page up/down button is the channel button.
I have the XR11 as well but it has both sets of keys: Page Up/Down and Channel. It read like yours only has the Channel keys.
With the Program Guide on your TV screen, what happens if you press the Skip Forward/Back buttons?
I read on another forum, a user reprogrammed the Channel button as Skip Forward/Back. This makes sense because I always thought the Channel button was useless. Most people just change channels through the Guide.
"...I opted to reprogram the channel up/down buttons to behave as skip forward/back buttons instead. As a side benefit, this remapping of the buttons counts as a page up/down when browsing the channel listing."