It depends on which 1020 version (RM-875, 877 or 876) you actually have and which 4G frequencies are supported by AT&T. RM-876 is 3G only but I would guess that most likely you have the RM-877.
Phone specs are as follows:
- 4G bands:
- LTE band 1(2100), 3(1800), 7(2600), 8(900), 20(800) - RM-875
- LTE band 2(1900), 4(1700/2100), 5(850), 17(700) - RM-877
According to this AT&T webpage - https://www.att.com/support/article/wireless/KM1008740/ the network's 4G LTE network is composed of the following bands:
- 700 MHz: Bands 12/17/29
- 850 MHz: Band 5
- 1900 MHz: Band 2
- 1700 MHz /2100 MHz: Bands 4/66
- 2300 MHz: Band 30
So if you don't have a RM-877 I'm afraid that your device will not work on AT&T's 4G network.