If the SIM card is not inserted properly, you might experience issues with network signal. To resolve this, power off your phone and remove the SIM card. Now re-insert the card carefully and make sure that it is inserted properly.
Sometimes a mobile device might lose the network connection due to a buggy software. Try restarting your mobile device, by removing its battery (if it is removable) or by long-pressing the sleep/wake button (and selecting restart).
Set your phone to select mobile networks automatically (iOS)
Usually, devices are set to choose and connect to mobile networks automatically. But, if a device is set to connect to mobile networks manually, you won't have network signal once you are out of the carrier's range. In order to resolve this, you need to set your device to select networks automatically.
Follow these steps to learn how to set your device to choose mobile networks automatically:
Go to Settings and tap on Carrier
Move the slider next to Automatic to the right
The device will start looking for networks automatically and it will try to connect to the network it identifies.
Update the mobile device's operating system and carrier settings
You need to update your mobile device in a timely manner, as the contains bug fixes, out of which one might be the network issue. Usually, mobile devices inform you when update is available, automatically, but sometimes you need to check for updates (and install them) manually.
If your device's IMEI number becomes nulled or corrupt for some reason, you might experience network connectivity issues. Open your device's dialer and type *#06#. If you don't get the IMEI number, which is a 15-digit number, but you get Null instead, there is something wrong with the IMEI.
If you are not sure how to fix this issue, I suggest you to ask a friend, or take your device to a repair store. Also, I advise you to check this useful article which shows the users how to resolve the IMEI issue, on Technobezz.