Contact network provider to check if the place where you stay has no signal of mobile network
Move to a place where your phone/tablet can receive stronger signal
Install your SIM card on another phone/tablet to check if there is problem with your device.
Access Safe Mode by long pressing power key > tapping and holding Power Off > tapping OK
If your device works well in Safe Mode, there may be some 3rd apps that cause malfunction
Please reboot your phone/tablet and check again if it works well