No service on iphone 6

I Recently just got my iphone 6 replaced because it was not charging with brand new chargers and i got my new 6 about a month ago and a week after i got the replacement i keep getting no service and the words no service is apearing in the top left corner on my other 6 i would have 4g lte in certain places in my town and now i never get any service at all and i never get more than 1x bars sort of thing and i can only call and text and use data on wifi any solutions or ideas that would be causing this problem. and it is updated to the latest version of ios it never worked on any of them