Would be good if we knew the procedure for sure. I tried to reset my Mini 2 for about 30 minutes, and it never "reset" to factory settings. My Wifi password did not reset to default no matter what I tried. Holding the reset for 5, 10, 15, 30 seconds with both power on and off. I eventually gave up.
Sounds like you weren't pressing the reset button. On the back of the M6+ there are several things that look like the reset button. I had to look at the diagram that came with the Users Guide to figure out which one was the right one.
Only one button on the Mini 2, (Assume Mini as well) - so no confusion there. It's funny because I did reset it one time before and it reset, but then I tried a week later and couldn't get it to fully reset. It looked like it reset, as the camera restarted, but the Wifi password remained (the password I picked).