I have a working module (at least the application with LoraWan & BLE communication), but I cannot update the module anymore. Nrfutil is not working because it seems to be some problem with entering into the boot mode.
I tried two scenarios to analyze what’s happening:
Set the device into AT mode before sending AT+BOOT. AT mode is set correctly, the devices resets after boot command but the COM port dissapears and never comes back until power toggle is applied. Then, the application starts normally.
Send AT+BOOT without being in the AT mode (I read somewhere, it should work). The device reboots but the application starts normally (no boot mode).
With the device still accepting AT commands and has normally working application code, the issue can be on the bootloader.
Unless you disable the AT commands via RAK_CUSTOM_MODE, you should be able to send AT+BOOT easily and upload FW.
Since after issuing AT+BOOT, COM port is lost, the bootloader code could have issue and the only way to recover it is using external tools like Jlink or RAKDAP1.
As you mentioned, it seems to be a bootloader problem. Unfortunately, I cannot check if it is possible to cover the module since I don’t have easy access to the programming pins.