New iPad not connect to 4G when switching from WiFi

My new iPad doesn't connect to 4G LTE when I switch it from Wifi network. The Cellular connectivity goes out whenever I switched the tablet to a WiFi or put it to sleep. I get 5 bars LTE again but there is not Internet connection.iPad says my cellular plan is not activated, but clearly it is activated. My wife and I both have the same new iPad but his device doesn't have got this problem. Then I