There can be certain reasons for an unavailable network on cell phones.
Probably you have removed the SIM card and not inserted it properly.
In case you are facing this while roaming then check whether your data roaming is turned on/off.
Might be some technical issue from your cellular network.
Solutions:
(1) Restart phone or reinsert SIM card to access network services Most of the time mobile network problems occur due to the technical faults in your mobile network service provider. In that case either you need to wait for sometime or reinsert the SIM card in your cell phone. Most likely it can be fixed by restarting your mobile phone also. This is the most universal reason why your mobile network is unavailable.
(2) Remove your battery Sometimes this happens due to battery issues. So switch off your phone for 2 minutes and remove the battery > reinsert it again after 2 minutes and switch it on again. Now you can expect to get back your cell phone network.
(3) Set mobile network Automatically If the above solutions doesn’t work out then try this. Check you mobile network settings by opening Wireless and Network settings. Now tap the Mobile Networks > Mobile operators and select ‘Automatically’. Your network will probably reverse back.
(4) Try the manual way (including on iPhone models) If the problem still persists then try searching for service manually. Go to Wireless and Network settings > Mobile networks > Mobile operators > Network operators > Search networks. Select your cell phone service provider or a supported network and check back again.
(5) Check Airplane mode Probably there is no mobile network due to your Airplane mode settings. See if your phone’s Airplane mode is turned on or off. If it is turned on then your phone signal will disappear. Most of the time it is mistakenly turned on due to finger touch so make sure your Airplane mode settings is turned off.
(6) You are out of coverage area Now if you are standing in a remote or out of coverage location where your cell phone service provider is not supported then it is obvious that you will be without any network connectivity. In that case your phone will not display any network signals and this can be sorted out only when you come out of there.
(7) Mobile network not available during roaming When you are in roaming it is obvious that you will face several connectivity issues. Signal issues are one of the major causes and to fix that you’ll need to check your Android phone’s data roaming settings. Go to Settings > Mobile networks > turn data roaming on.
(8) Check your device’s firmware update Many times signal issues occur due to downgraded software. When your Android phone has an update available it will send a push notification to you. In case you don’t receive any such update then you’ll need to check that manually. To check it, go to Settings > About device > Software update > Check for update.
(9) Try Factory reset If it is still not fixed then probably your phone has get into a critical stage. Now try factory resetting your phone but make sure before doing it you back up all your important data from the phone. Go to Settings > Backup and reset > Factory data reset.
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