Iphone 5s No Service

Recently started using the iPhone 5S and frequently facing the network drop issue wherever the signal strength is one or two bars . However am not facing this issue in the open area and getting full strength with all four bars of tower. I will get the tower when toggle between the normal to flight mode and vice versa. Completed all the basic troubleshooting in my side and people in the service center are not able to reproduce this issue and advised me to use the new sim. How to rectify this issue and where the problem exactly is ? Even I compared with the other models of Iphone and Android devices which are working fine with good coverage and no dropping issue identified.