Does the iPad Air automatically switch to 4G if Wi-Fi is down? My mom has an iPad Air with a T-Mobile data plan.  The iPad Air is configured to use the Wi-Fi in her home.

She recently noticed some pop-up messages on the iPad Air saying that there's no data left on the cellular plan.  She gets 200 MB of data every month from T-Mobile, so I guess she used up all 200 MB this month.  But she never takes the iPad outside the house, so she doesn't know how she ran out of cellular data.  The only thing I can think of is that maybe the Wi-Fi was down and the iPad Air automatically switched to the T-Mobile cellular signal.  She watches movies a lot, so that would certainly use up 200 MB of data pretty quickly.

So does the iPad Air automatically switch to the cellular plan if the Wi-Fi is down?  What if the Wi-Fi router is up but the Comcast internet is down?
It works the other way around but the result is the same. 

By default it uses the cellular network unless there is a Wifi network. If there is a Wifi  network I will use it to send and receive data.