If the 12vdc battery is toast, you won't be able to use the vehicle at all! All of the lights, computers, locks, etc., rely on the 12vdc battery. Once the vehicle IS in ready mode, the main battery comes into the picture and will try to maintain that 12vdc battery, but that only happens if the car is in ready, or can get there. Well, there is some stuff going on while charging with an EVSE, but charging the main batteries can't happen unless the 12vdc one is sufficiently charged, either. FWIW, there is a bypass, trickle charge emergency mode to try to get the 12vdc battery voltage back up, but it can take awhile...until that happens, you can't charge the main batteries or drive the thing.
People have reported all sorts of weird issues when their 12vdc battery was getting marginal. If yours is getting more than 4-years old, it might be time to consider replacing it before it fails. I should do that to mine, but haven't yet.
2011 535i x-drive GT, 2014 i3 BEV
Soon (hopefully!) A 2021 X5 45e will replace the above