The LG UltraFine 5K has a 2560 x 1440 60Hz timing (234.590 MHz) which should work. Try connecting it after the computer boots?The 2012 doesn't support 4K so might be the issue. It is not listed under supported Macs even with the TB2<>3 adapter. https://www.lg.com/us/support/help-library/monitor-connecting-to-a-mac-CT00020679-20150911473158