Maybe your mobile device experiences software-related issue which prevents it from displaying the strength of the network signal accurately. Try restarting your mobile device, by removing its battery (if it is removable) or by long-pressing the sleep/wake button (and selecting restart).
If the network signal weakens only when you are holding your device, it's possible that you are covering its antenna. Usually the antenna is located near the camera or near the USB (charging) connector. For example, on iPhones the antenna is located near the Lightning (charging) connector.
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.
Maybe the protective case weakens the network signal. To check if this is the case, remove the protective case and check the network signal strength. If the network signal is stronger, either use your device without a protective case, or purchase a different case.
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.