Please provide more information about your application. Do you use Softdevice or you are using NCS SDK ?
We are not familiar with the GSM OTA you referred to in the link. So we don't know how it works. But I would assume it would use UART to interface the GSM modem with the nRF52840. We do have example for open bootloader with UART/USB in the nRF5 SDK.
Could you let me know which firmware is running on the nRF52 ? Do you use NCS SDK or nRF5 SDK ?
I'm not very familiar with the SIM7600 so I can only give some suggestion.
I assume there is no MCU on the SIM7600 ? If it's the case you would need to receive the image in the application and store it in the internal (or external) flash. After that you switch to the bootloader to replace the image. Please answer the question about the SDK so I can give the suggestion.
If you are nRF5 SDK, you can have a look at the bootloader example.
There are 2 options, secure bootloader and open bootloader. In both version there is no encryption. The open bootloader is the version without signature verifying. Please have a look at the documentation here.
In your case you would need to implement background DFU. Meaning you receive the image in the application and only switch to the bootloader to update the image. Basically you need to enable NRF_DFU_SETTINGS_ALLOW_UPDATE_FROM_APP in the bootloader sdk_config.h .
You then need to implement the part the receive the image from internet in the application. We have an example that my coworker Vidar made here. Note that the example is provided "as-is" you would need to study the code and make the modification to fit with your needs.
