If the «no network» icon appears on the smartphone screen, the device does not catch the signal or the mobile data icon has turned gray, the reason may be both in the phone and in the SIM card.
SIM card malfunction — one of the most common reasons, you can verify its serviceability by moving the card to another phone.
If a signal appears in a new device, the problem is not in the SIM. However, if its surface is damaged, there are cracks, oxidation of the contacts, or the card has been used for more than 3-5 years, it may have failed. Simply cleaning the contacts from dirt and dust may be enough to restore operation.
The problem may be due to damage to the SIM card tray. If the phone supports two SIM cards, the phone may fall or be carelessly handled.
It is also possible that the phone is undergoing maintenance or operator failures. In such cases, many users in a certain area lose connection. Operators usually report such problems via SMS or social media.
In dense urban areas, underground spaces, or outside the city, the signal may be unstable. Switching from 4G to 3G in the device settings often helps restore the connection.
If the device is outside of network coverage, switching to slower communication standards such as 3G or 2G may solve the problem. Otherwise, you need to change your location.
Sometimes the reason lies in the phone's settings. For example, the network settings may be reset. To restore, you must manually specify the operator in the mobile data settings and select the network type 4G/3G/2G (auto).
A network failure can also be caused by incorrect firmware. Installing an incompatible version of the system or an error during the flashing process often leads to incorrect operation of communication modules. Restoring the factory version of Android will help solve the problem.