Wi-Fi networks will almost always provide faster and more reliable access to the Internet, but cellular access is very convenient when Wi-Fi is not available.
Sometimes, the Chromebook will connect to the cellular network automatically, but very often it must be done manually. When you try to login, you might see a window pop up saying "Network not available." Click on the name of the carrier (it will be either Sprint or T-Mobile).
At the prompt below, click to highlight the name of the carrier and then click Connect.
Another way to connect is to click the lower right corner of the screen in the area showing the time and other icons. You should see the name of the carrier and you can click on it to connect. In the image below, the Chromebook detect that the T-Mobile network is available, but it is not connected (as indicated by the little x next to the signal bars). Click the name of the carrier to connect.
