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LoRa-E5 Module, Grove - LoRa-E5, LoRa-E5 mini, and LoRa-E5 Dev Board are programmed with the same AT Firmware and bootloader. If you find ‘C’ character comes out from serial at baud rate 115200, that means your module is running in the bootloader.
We use PB13 to control the module to stay in bootloader(for DFU) or jump to the AT application. When PB13 is HIGH, the module will jump to AT application after reset.
What is more, the Factory AT Firmware is programmed with RDP Level 1. If you want to develop your own application, please remove RDP first with STM32Cube Programmer. Note that regression RDP to level 0 will cause a flash memory mass erase. That means the Factory AT Firmware can not restore.

from: https://forum.seeedstudio.com/t/how-to-program-a-lora-e5/257491