First try to change your SIM card, meaning use any other network may be your network is having issue. If so, then go to your service center and get a brand new SIM.
• Switch your mobile network settings from LTE/WCDMA/GSM to Auto. Now, wait for few seconds then reboot your device.
• When your device is rebooted. Go to Settings.
• Network connections.
• Then More Networks.
• Now Mobile Networks-> Network mode.
• After getting in to the network mode switch back to LTE/WCDMA/GSM mode.
• Again reboot your device.
• This method worked for most of people so you should give it a try.
After performing all the above instruction and if your mobile data connectivity problems still remains the same. Try toggling airplane mode to get it connect. If none of this worked for you ,then go to your Samsung service center they’ll fix this problem, if not then you will get the new device.