Since then, no factory.bin has been sighted.
Please mind that only approx. 30% of all OpenWrt supported devices do require the presence of a factory.bin for initial installation.
Have you followed the installation instructions in the git commit linked above?
N12+ B1 is a different model... It's the one that I have and it sadly isn't compatible with OpenWrt for the time being D:
I'm not sure if anyone had success flashing to that device just yet but there probably is hope, please refer to this reddit comment and wish you luck on this one!: