Bootloader for LoRa E5 Module

There is no additional Documentation for the bootloader in the LoRa E5 Module available. Only Documentation available is it enters with PB13 LOW at Reset and it transmits a C Character every 1s at 11500 Baud. No protokoll description, no hint of its use with active Read Data Protection, and no sourcecode of the bootloader to use it after removing the RDP ( and erasing the Chip )
I assume a bootloader to use these Modules ( and the Boards with USB-Serial ) like an arduino without another Programmer would be highly appreciated.
I hope this was in the correct category ?

Thank You
schimmi