For people having external display issues on boot camp (Windows 10) I just solved my issue of always defaulting to minimum resolution no matter what setting I picked by doing this:
Manually went to AMD and installed the graphics drivers first
Control Panel->Display->Advanced Display Options->Select external monitor->Display adapter properties->List Modes
I had the same issue for a long time. At first, 60Hz worked. Then something happened and I'd only get 30Hz on my 4K Lenovo ThinkVision 27".
I stumbled upon this article and simply went to Display settings and hit "Resolution: Default for display". Immediately the display switched to 60Hz. Then I could reapply my Scaled text size setting and 60Hz remained.