Q: Iphone 6s plus - keep getting Network Error - Failed to connect

Settings - General - Reset - Reset Network settings you will need to re enter WiFi passwords

Also

Look for a carrier update

Try to pop out the SIM card - wait for a bit and re inset it back in

Remove or switch the SIM card in your iPhone or iPad - Apple Support

Try turning Cellular OFF, Airplane Mode ON, the a forced restart, and then turning Airplane Mode and Cellular back ON.

Forced Restart. Hold down the Home and Wake/Sleep buttons at the same time for about 15-20 seconds until the Apple logo appears. You will not lose anything.

Will this be true with iphone 6s not able to connect to network data?

No guarantees, all you can do is try it. If it does not work other options can be pursued.