Flashing the AT&T radio probably won't help. GSM stands for "Global Standard for Mobile Communications". It is a standard and does not differ from carrier to carrier.
Additionally, the XT1053 supports 1 extra UTMS band, and lacks 2 LTE bands that the XT1058 has (but this doesn't affect you because AT&T doesn't use those 2 bands, and the XT1053 DOES support all the bands AT&T currently uses).
You can flash the XT1058 baseband IF you have an unlocked bootloader, but you probably won't get any support here. We don't like to support phones with mismatched software/basebands -- they just cause too many problems.
If you insist on doing it (assuming you are unlocked), you would flash fsg.mbn (modem) from the XT1058 SBF, and you would need to:
fastboot erase modemst1
fastboot erase modemst2
I urge you not to do this. Flashing a mismatched radio will often result in WORSE battery life, and you may not even get a signal on some or all bands.