Unfortunately it is not possible to upgrade your Fairphone 1. The frequencies, that are supported, depend on the baseband modem used in a phone (not on the antenna). Since there are no modified versions of that part for the US market, your FP1 will stop to work when AT&T turns off 2G.
I see two possible options for you:
- Switch provider to one that provides service on the frequencies supported.
- Sell your FP1 and buy a FP2. (Working FP1s sell at quite high prices, some even at the original price.)