Data connectivity Issue,LTE sign vanished by turning from OFF to ON

Hello Sir,

I have a serious problem in Lenovo k8+,when I tu ON the Data connection from OFF state,The LTE sign which shown in between the 4G and Network sign option is disappear by doing first OFF the data connection and then ON the data connection,

this problem also arises when you disconnect the WiFi connection.

To get the data connection back,I need to tu the flight mode OFF-ON. 

Can you suggest the Solution of this problem.

I tried both Mobile Reset & Network Reset, but nothing happened.

Thank you.