The number of bars has nothing to do with anything, unless you're comparing them on the same phone. Not the same model phone, the exact same phone. If you get 2 bars here and 3 bars there, the signal is stronger there. But the bars aren't like 20db/bar, or anything like that. A phone could get 5 bars on a barely usable signal, while another phone of exactly the same model could be completely usable with a one bar signal on the same carrier.
If the phone isn't working on a particular carrier, use an app like LTE Discovery to see what your 4G signal strength is. If it's less than ((a higher number than) about -103db, you're using the wrong carrier for where you are. Not all carriers cover all areas equally. (The worst thing to depend on is carrier maps. (I know a house that gets absolutely no measurable signal at all from Sprint, due to what's called the "knife edge effect" from the higher ground across the street, but Sprint's map shows solid coverage in that house. And it is - in the yard, on the front stops ... but not anywhere in the house [except for a tiny bit of reflected signal in one window pane in one window at the read of the house].)
So measure the actual signal strength. If it's strong, and you still can't use the phone, that's the phone's fault (And a bad receiver or antenna is covered under warranty.) If AT&T just has weak signal where you are, they have weak signal - and there's nothing you can do about it. (There's a whole town around here that AT&T doesn't cover. Most of the people living there re on Lifeline, which AT&T isn't participating in, so they don't care. But Sprint covers the town pretty well.)
The first thing in setting up for a cellphone is determining which carrier covers the areas you need covered. The best phone in the world, for free, with no charge for service, is totally useless if you have no coverage. Then you choose a phone that can be used on that carrier. Choosing the phone based on which carrier sells it can be throwing money down the drain.
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