Same here so we bought a Wilson Antenna cell phone booster. It works now. Best By carries them and so does Amazon.
At a minimum you will need some type of roof mounted antenna connected to the booster. A booster with a directional outdoor antenna will provide the best results. (But they aren't cheap)
Have you ever tried a different phone? Some models work much better in marginal areas than others. I am assuming you are not using any type of "Advanced Calling".(VoLTE, HD Voice, etc . . . . )
Any AT&T coverage in the area that you know of?
You need something like this, but will need to know which way to point the antenna. The only downside is that the 3g & 4g antennas may not be in the same direction.
I live in bumtillie and have days with 2 bars 3G 4G 850MHz at T Verizon Cellphone Signal Booster Repeater Yagi Antenna Kit | eBay
Check that out and there are many others, some passive, some omnidirectional and some directional.