Here are a few things you can try for resolving the connectivity issue on your phone. The first thing you can do is to switch your mobile network settings from LTE/WCDMA/GSM to Auto and reboot the phone after few seconds.
Once rebooted, go to Settings> Network connections> More Networks. Choose mobile network and then choose network mode. Now switch back to LTE/WCDMA/GSM mode followed by rebooting.
This would definitely work out. Else, check with the customer care or get a new sim.