The iPhone 5 doesn't have an external antenna plug. If you see something claiming to extend your atena's range that's not a plug-in repeater it's a scam.
The only other solution is to get a AT&T Micro Cell for $100 (they lose money on every one so it's a good deal), which is a minature cell tower you connect to your home internet. Or you could get a pricey cellular signal repeater, which would run $300.
If everyone else is getting a good signal consider switching carriers, and you could report the poor signal to AT&T - they might fix the issue in a few weeks.
There's no cheap solution.