Hi, atlasgeo, maybe I can help you. Yes, you're correct, there are no Mac drivers for that monitor. However, the HDMI version does make a difference. There are two HDMI sockets/ports on the monitor, a 1.4 and a 2.0...... 1.4 supports up to 1080 and 2.0 does 4k. So, maybe make sure it's plugged into the correct socket/port which should be labelled, and yes, I know, getting in there and unplugging and re-plugging in the cable is a pain in the butt, but they're all kinda like that. Luckily you only have to do it a few times (whew !). You should also only unplug and re-plug your monitor cable when both your Mac Pro and your monitor are turned off. the Rankie cable says it supports 4k and audio up to 7 channels, so I'm guessing it's HDMI 2.0. After you have connected the HDMI end to the proper socket on the monitor, check under system preferences then displays. If you hold down the option key while clicking around, detect displays should come up, and you should be able to choose scaled resolutions instead of just "best for display". Anyways, atlasgeo, give all that stuff a try. You also might want to reset smc/pmu and zap pram on startup as well. Couldn't hurt...
Ohhh.....I just got it..you have an HD 5770 video/graphics card. the max resolution of that card is about 2k and it supports HDMI 1.3 and DisplayPort, plus Dual Link DVI. Yes, you will need an active MiniDP to HDMI adapter for that. Kanex is the recommended brand, I believe.....
well, that's all I have on that
john b