iPhone 11 Pro Max - No bars!

It isn’t a defect. If you are out of range of your network (such as the interior room of a building or an elevator) and the phone loses a network connection it will wait a few minutes before trying to connect again, because the process of finding a network uses a huge amount of energy, and would run the battery down quickly. And putting the phone in Airplane mode for a few seconds starts the search process over. If you are connected to Wi-Fi you should enable Wi-Fi calling (you need to call your carrier if your phone doesn’t have it), so you can make and receive calls even if your network signal is too weak or non-existent.