From the ATT U.S. support page on BYO devices:
You can bring your own phone or tablet to AT&T with no annual service contract, if your device is unlocked - not programmed to work exclusively on another wireless network, and will work on one of these GSM network frequencies:
3G UMTS network in the 850/1900MHz bands
4G LTE network AWS/700/850/1900MHz bands
From the Microsoft site referenced immediately above (microsoft.com/en/mobile/phone/lumia640-lte-dual-sim/specifications/)
Cellular connectivity
GSM network: 850 MHz, 900 MHz, 1800 MHz, 1900 MHz
GSM max data speed DL: EGPRS 296.0 kbps
GSM max data speed UL: EGPRS 236.8 kbps
WCDMA network: Band 1 (2100 MHz), Band 2 (1900 MHz), Band 4 (1700/2100 MHz), Band 5 (850 MHz), Band 8 (900 MHz)
WCDMA max data speed DL: 42.2 Mbps (Cat 24)
WCDMA max data speed UL: 5.76 Mbps (Cat 6)
LTE FDD network: Band 1 (2100 MHz), Band 17 (700MHz), Band 2 (1900 MHz), Band 20 (800MHz), Band 3 (1800 MHz), Band 4 (1700/2100 MHz), Band 5 (850MHz), Band 7 (2600 MHz), Band 8 (900MHz), Band 28 (700 MHz), Band 12 (700 MHz)
TD-LTE network: Band 38 (2570-2620 MHz), Band 40 (2300-2400 MHz)
LTE max data speed DL: 150 Mbps (Cat 4)
LTE max data speed UL: 50 Mbps (Cat 4)
I'm not very knowledgeable about frequencies either, but it appears the 640 LTE as listed on the referenced site should be compatible on ATT in the U.S.