What is the AT command to enter the bootloader?

The AT command is: BL. Note that the bootloader runs at 115200 baud so it is best to first change your baud rate to 115200 (ATBR7), do a write to flash (ATWR), and then enter bootloader (ATBL). After you update the firmware, the default baud rate of the application will be reset to 9600 so after exiting the bootloader you will need to change the baud rate of your terminal emulator to 9600. Type “AT” with no command to see if it responds with “CMD OK” to verify your current baud rate settings are correct.