According to Phone Arena, the LG Fathom can operate at the following GSM Frequencies:
850, 900, 1800, 1900
AT&T uses: (according to
Wikipedia)
850 MHz UMTS/HSPA 3G 1900 MHz UMTS/HSPA 3GWhich means that it should technically work. You may need to force the phone into GSM\UTMS mode and make sure that UTMS is enabled. I know it's possible to set it to GSM only which disabled 3G for battery saving purposes.