iPhone 5S Signal Issues

Basically, the problem that I am facing started to occur after installing iOS 7.1 whereby the cellular signal would at random times drop down to 1 bar. When this happens, I am unable to call or receive any text messages. A temporary fix would be to toggle airplane mode on and off for a few seconds or even restart the phone. However, the problem would re-occur again at a later time.

Other options that I have tried but were unsuccessful include:

- Restoring the phone to factory settings

- Resetting the network settings (countless times)

- Changed the SiM card to a new one

- Got a replacement iPhone 5s - problems started again after updating to iOS 7.1

- Updated to iOS 7.1.1 hoping for a fix, but the problem still remains

I also tried inserting the same SIM card on an iPhone 4s and it worked flawlessly, so the problem seems to be related to the iPhone 5s.

Additional information: I am currently experiencing this issue on an iPhone 5s Champagne Gold on a Vodafone Malta mobile network.