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.