I've had my 5s for a few months now and only once has 4G ever worked. Every time I try to use it my signal drops and it just displays "no service". If I reset network settings the option for 4G is on but its only using 3G and when I'm using the internet it will randomly lose signal then gain it again. Anyone ever have to deal with this before? Model is a iPhone 5s MF356X/A (back of the phone says A1530)