HI,
Try a different SIM or try swapping SIM.
Run SMMI test to check if this is related to hardware
How to Enter SMMI_TEST (test hardware)
Go to Calculator=>type " .12345+= "
you will be entered to SMMI_TEST
Update your phone's software to latest version if not.
Last step would be:
Kindly visit the nearest service center:
https://www.asus.com/in/support/service-center/india