Since Notify 8.11.1 version you can get your band working also without official Mi Fit app installed.
1) Open main left menu and choose "Replace with a new band"
2) Follow the wizard to re-pair your band
3) When asked for the auth key follow the tutorial to get it (freemyband.com method)
4) IMPORTANT!! Do NOT unpair the band from the Mi Fit app before uninstalling it
5) After you have uninstalled the Mi Fit app, and come back to Notify app, the auth key will be loaded automatically (compatibile only with freemyband.com method)
Credits: www.freemyband.com