How can i fix No Service error ?

My mom using an iPhone 6s. In-network areas, suddenly it is showing No Service. I have tried to update the phone, and I could not update it, so I have connected with the computer and updated it. Even after updating the phone, it keeps showing No Service. So, I tried to reset the network setting; normally, when we reset the network, it restarts the phone, but it was not doing anything when I did it on this phone. After that, I restarted so many times. But it did not work. I tried to check the mobile network, but I could not see Mobile Data as there it is showing ERROR in front of the Mobile Data option.

I tried to change Simcard, but all sim cards that are okay on other devices are not working on this phone.