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.
Note: If the factory reset didn't resolve the issue, consider doing a hard reset, but make sure to back up all the personal data(photos, videos, sounds) before doing the hard reset, because all the data stored on your device will be deleted.
Subscribe to a different operator (carrier)
If you live in a rural area and you are facing network connectivity issues constantly, consider subscribing to an operator whose coverage is better in your area and that has a network signal booster installed in your area.