Hi @Lincoln,
What is the SDK version that you are working on?
Are you working with the internal or external flash?
Did you disable the low power mode in the image that you are sending?
However, I followed the same process working with the SDK version 2.6.12 and the external flash as default.
Is it possible that you could use the same S19 file created with the OTA ATT example?
Note: After the OTA process you have to wait some seconds for the bootloader to copy the new image to flash.
Regards,
Mario