iphone6 continous NO SIGNAL

For the past week, I keep getting NO SIGNAL on my phone. I have the latest 8.3 update. A few seconds a day I may get one dot of signal but this is pathetic. I took to At&t store and the Network settings has been reset several times. Off/On has been tried. We thought it began working but after I left it returned to NO SIGNAL I spent another hour with my provider tech support a few days later to no avail. I think it's the phone. Can someone please help me with this?