@Joolminni: To begin with, please swipe up from your Home screen and head to Settings > Software update > Download and install, and check for any available updates for your phone. Now, head to Settings > Connections > Mobile networks > Access point names, and check to make sure that your network plan is registered here. If it isn't, tap the 3 dots in the top right > Reset to default, to see if this helps. Tap the Back button > Network operators > Select automatically, and make sure that this is toggled on.
If the problem persists, power off your phone, remove the SIM card and give the gold contacts a rub with a dry, microfibre cloth, then re-insert it and power the device back on. When the phone has finished booting, check the top left corner of the lock screen, where your network details should be visible.
To further troubleshoot the issue, please head to Settings > Connections > SIM manager, and make sure that your SIM is recognised, and toggled on. Also, you may wish to head to Settings > General management > Reset > Reset mobile network settings > Reset settings. If you continue to see the network notification, then please try your SIM card in another compatible phone, as this will help to determine if the problem lies with the phone, or your SIM/network provider.
