Well i can describe in general. But keep in mind I do not take any responsibility of any kind. Even more this will break stock Shelly app, it is likely trivial to restore but i didn’t try. First of all you will have to disassemble a device to get to usb c hidden inside. When you get to blue main board inside, unscrew it, remove the microphone on top and make sure not to destroy cable. Then pull top part out while making sure not to destory FPC’s on bottom, if you are not brave enough (or you dont want to risk) just disconnect both FPC’s on bottom (one is LCD another one is temp sensor). Then follow instruction in mtkclient/README.md at main · bkerler/mtkclient · GitHub to install mtkclient, it is avalible for Linux and Windows after doing so run:
/path/to/mtkclient/mtk rl dump
Make sure folder you are currently in have 8GB free space. While that is running connect usb C to a cable to pc and device, after that connect power to device. After that you should see backup progress, it will take a long while, around 20 minutes. Disconnect power, run
/path/to/mtkclient/mtk da seccfg unlock
Connect power back, after that command finished, disconnect usb and reconnect power. Device should show “Erasing” for a moment and reboot itself, after boot you will see shelly app with error message, but this time navigation bar on bottom and status bar on top will appear. Swipe from top to open quick settings, in there on bottom right there is a gear icon, click on it to open android settings, got to the bottom and open “About tablet” click on build number 7 times, go back and open an entry “Developer options” that did just appear. Here you can enable USB debugging, and use adb normally. So you can install apps. To restore shelly app you likely need to copying stuff from your backup, specifically a userdata. It is unencrypted so should be easy. Sorry for bad English. Anyway i will work to make process simpler. And i will likely build custom rom based on newer android, and one that will be easier to work with.