I changed my bank and it seems that the only possible way of accessing their online website is having installed an app called "tan2go dkb".
I've read that it is not possible to use it with a rooted phone. I won't change my phone just because the bank says so, so I tried to use an old motorola moto g 1st gen.
I erased everything, and flashed the stock rom, I also tried to flash a stock firmware. After flashing the new stock ROM I install some apps to tell if I'm still root and they say I'm not.
So, which kind of check are they doing in order to tell that my phone is rooted even after flashing a clean stock ROM?