NO SERVIC ON A SAMSUNG GALAXY S7

Issue
No service on your Samsung Galaxy S7/S7 Edge 
Restart Your Device 
Go to Settings > Backup & reset and tap on Reset network settings

This will erase saved WiFi passwords, so make sure to reconnect to WiFi after this step
Tap on Network Mode. Select the second WCDMA/GSM (auto connect) option
Then select Network operators and tap Search networks.
Once the search is complete, select your Carrier. Test your device.
If issue persists than install a fresh OS on your device