sim not valid

hi sir,mam

I erase all the content and settings from the iphone x by going to settings . Then, when my phone restarted it was saying SIM NOT VALID . Further i visited apple service station i said my problem to them . It was new for them they said we did not seen this problem in their career . Next to it they chat with apple and sent my iphone x to apple in Bangalore (India). They gave me iphone 8 for use till my iphone not received from apple . But when i came to home and inserted the sim to iphone 8 again same problem repeated . what are the causes of this ? how to curve this problem ? please help