The 5GHz WiFi channel dissapears if it is given the same name as the 2.4GHz channel. Login to your gateway. Then, select Gateway > Connection > Wi-Fi. Are the names for the 2.4GHz and 5GHz bands the same? They should be. Click the edit buttons and verify that the same SSID is used for each.
There is no need to have different names for the 2.4GHz and 5GHz connections. In fact, if you wish to use xFi Pods when they are available in your area, you will find that a prerequisite is that both names be the same. This way, the correct speed connection is automatically selected. Why manually select the connection when the system can do it for you? Newer iPhones and iPads will connect at 5GHz if the signal level is high enough.
You may also check and change the names and SSIDs with the xFi App if you are using an XB3 or XB6 Wireless Gateway.