Could be the phone, but could be some change in their local coverage that caused the problem since you did have it at one time but no longer.
We had AT&T several years ago. We made an annual trip to south central Colorado every year. We got very weak service at the campground (about 5 miles from town) but great signal in town. But others with AT&T got reasonable service at the campground. Upon returning home we went to an AT&T store, explained the problem, and they suggested new phones (surprise, surprise), and recommended specific models. Well the next time we went to the same campground, not only could we not get signal at the campground, we could not get it in town either. Called AT&T and explained the problem. Their first reply was that they do not provide service to that area. When I explained that I had got service there previously (as noted by ATT on the phone screen), they could not explain it. I asked for different model phones to replace the new ones that did not work where our old ones did. Then their basic reply was, sorry, you were the ones who decided to buy new phones. But they would be glad to sell us different new ones.
Went to Verizon and never regretted it. Much better coverage at that campground and every where else we have ever been.