Three things:
1. I don't know what "third-party website" you got your firmware from. I generally download any that I need from
https://pgaskin.net/KoboStuff/kobofirmware.html .
2. The SD card to use isn't just any old card: you need the one from inside your e-reader, and it has to go back into the slot from which it came.
3. Where on your SD card did you put it? It has to go in the .kobo folder, not in (for example) the root directory.