Hi dgeojoseph,
We would suggest that you try:
(1.) Settings > Network & internet > SIM card> Disable and enable the SIM card
(2.) Settings> Network & internet > Mobile network > Preferred network type> choose "2G/3G/4G" again.
(3.) Power off your phone > reinsert sim card > turn on the phone
(4.) Turn on airplane mode > wait for 3 seconds > then turn it off.
(5.) Try using another sim card by a different operator
(6.) Try resetting the network settings: Settings > System > Reset options > Reset WiFi, mobile & Bluetooth)
If this problem still appears, what we would advise is that you have the unit sent in for an assessment and servicing where necessary.Sorry for any inconvenience this may cause.The link below will help in finding your local service center: https://www.asus.com/support/Service-Center-All-Countries/