Did you make it work? I am having the same problem!
Thank You for the kind interest.
Well: at the moment I have not being able to solve this issue.
I am continuing the use of Mac Pro and its OS High Sierra and the original little graphic´s card .
Nevertheless , so to say , I have been suggested to wait for it and try buying some used graphic card , at the next local electronics fair , in order to catch the right used one , which , maybe , could etc etc etc Lol !
for example : I am now , perhaps , discovering that , with help of this site of Ours , - that graphic card RX 560 Radeon " ROG STRIX GAMING would be an Eastern production , not compatible with other type of computer ( or s.th. like that ... lol)
Therefore I was in my way to reconsider the right and appropriate MSI graphic card RX 560 Radeon: as I have been told that it , too , is of Eastern production , it should, therefore , not be able to solve the issue
Or else , which should become a real surprise to me , I should only think and conceive about an internal computer element which is wrongly behaving and functioning wrongly functioning ... Whoknows ?
Please : let us know how and where to solve those kind of little problems we all have !
Best Regards