You can use 123456 to login.
If it repeatedly asks for the pin, try rebooting your phone.
The JBD BMS pin was an uncompleted feature by the manufacturer. The app never actually uses it, you can put in any 6 digits and it will work. We only tell people 123456 in case they someday do implement it, at least then we are all on the same page. In fact, the iOS App never asks for a pin and logins pretty flawlessly, even on our iPhone 5 for testing.