I installed my new 4G Sim yesterday and had it activated. All was good. Today however, I have no signal for most of the day. It flicked on momentarily for a minute or so at a time but has had no service for 95% of the day. I tried switching the phone off and back on and was again asked for the sim activation code. when I enter that the phone is on but still with no service. I also tried switching back to 3G setting with no luck?