iPhone se cellular signal drop

Hi, I have recently bought an iphone se second hand and it's warranty period is expired. This phone signal strength is always 3 bars at home so I can receive and make calls, send text etc.

But when I travel in bus or walking to office the signal drops to zero and then the signal never comes back even after restarting the device.

It only comes back if I swap the sim to another phone where it works great and after that I put the sim back to this phone.

I did everything network reset, ios restore, latest ios updates etc.

Any ideas what could be the issue?