I've use these converters for the past two years and they have worked well for my monitors
After searching online, only thing I found was a guy that claims this adapter works https://www.amazon.com/dp/B00K0UDJFI/ref=cm_sw_r_cp_tai_XR0PzbF6E4NCE
It sounds like the HDMI port is only physically connected to the onboard Intel graphics
This is the adapter I used: https://www.amazon.com/gp/product/B00K0UDJFI
I have no personal experience with this, but:
/u/cybereality once recommended the Cable Matters Mini DisplayPort (Thunderbolt™ 2 Port Compatible) to HDMI Male to Female Adapter - 4K Resolution Ready. I see people in the answers section saying it works well.
Actually any cable like https://www.amazon.com/Cable-Matters-DisplayPort-Thunderbolt-Compatible/dp/B00K0UDJFI/ do work since 4k@30hz (hdmi 1.4) is enough to handle the rift. I know i have no issues with my standard passive hdmi-dp adapter
I use this one. Works fine for me.
Cable Matters Gold Plated Mini DisplayPort (Thunderbolt Port Compatible) to HDMI Male to Female Adapter - 4K Resolution Ready https://www.amazon.com/dp/B00K0UDJFI/ref=cm_sw_r_other_awd_sVlQwbQ9Z3X79
Same boat here. Bought this monitor yesterday. Can't get the resolution to match with the same adapter on a 2011 MacBook Air. We need to buy a better adapter. These's are my next purchases in an attempt to fix the resolution issue.