There are two frequencies on WiFi - 2.4 GHz and 5 Ghz. The 2.4 GHz channels go no higher than 11. All the 5 GHz channels are higher than that. Your gateway will typically pick the least congested channels automatically, but you can also manually select the settings on your gateway by using the Admin control panel or at Xfinity xFi:
If you have a personal router, check your device's documentation on how to change the WiFi channels.