I had this same frustration. I worked with Apple, Wacom, and StarTech to get to the bottom of this as there is a lot of misinformation.
My 2015 iMac almost all adapters to the display port worked just fine. But my new 2017 iMac I just got working 5 minutes ago after weeks of trial and error of different connections and combinations.
The problem with the 2017s I found out this week (Dec 21 2017) that nobody at Wacom has tested a Cintiq with a 2017 iMac. They don't even have access to one. Their official policy was to suggest customers buy an older Mac with a display port as they didn't have official adapters and only heard rumours of some that work (but really only work with 2016 and older)
Apple tried their hardest to resolve, but they don't have an adapter that will will work at this time (as we went thru them all) and to be fair, it is Wacom's problem.
Wacom suggested a Startech USB-C to DVI adapter which SHOULD work, but it seems, the chip inside does not work with 2017 iMac as some Amazon reviewers have had the same result as me.
I tried going through HDMI.... DONT! It becomes really laggy and leaves the mouse unusable.
I didn't try VGA as it would be a last resort with poor resolution.
In the end, I called StarTech directly and they worked with me.They were even so helpful to tell me that if there wasn't a suitable adapter they would work with me for a solution and alert the engineers to adapt one.
StarTech suggested Going from display port (the one with the cutoff end) to usb-c. I did it with their latest adapter as the DTK-2400 Cintiq has a dormant 20 pin display cable stored in the base already (you can get access to it with 4 screws), but you can also just order the replacement cable for a little more $.
Works perfectly and now I don't have a 3k boat anchor next to my new iMac.
So in conclusion- there are probably other connectors that work, but beware as Wacom has not tested them with 2017 iMac as of yet. StarTech Usb-C to DisplayPort (20 pin). Model : CDP2DP
I have personally let Apple Tech know the solution. I also let Wacom know how disappointed I am in the lack of support, interest, or sense of urgency to inform their tech, suppliers and customers. Their own supplier had given up and recommended PC for new Cintiqs. 6 months is an eternity to wait for a solution.
-Now back to the ol' drawing board (literally...and finally).
Funky Chicken. www.poultrypower.com
