I have an old 2016 iPhone SE but don't recall exactly what happened when I installed the My BMW app. I recall that after I logged on to the app using my ConnectedServices login credentials, the app asked me to create a PIN. I then set the app to use Touch ID to log on so that I don't have to enter my PIN.
Using the ConnectedServices login credentials, the app should recognize that your i3 has already been registered. Sometimes, apps need to be given another chance to download information about the person logged in. Maybe try logging out of the app and then logging back in by going to Profile -> Settings -> BMW ID and then clicking the "Log Out" button.
2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE