I made the same mistake. What you need to do is to flash a d802 4.2.2 rom (I used the UK one), and it will flash, but be really buggy. You need to rush to the software information screen and enable developer options by clicking on the build version. It might reboot before you can, so you need to be patient. Enable Airplane mode, sometimes it helps. After you enable Developer, you need to toggle Android Debugging.
When that's done, you need to root and flash a recovery. After that, you're basically home free - flashing the d805 radios will make the phone stable, or you can sideload CM11 or another AOKP rom (plus the radios again) to have a working phone.