Hi there,
What firmware is on it , in the About screen?
I think there is a way to reset it while connected and it should auto detect the rate and adjust.
some units didn’t do this initially.
We had one on a CAD system and it was very frustrating to change resolutions and sometimes it would just not display. the DP connection worked without issue, BTW.
The HDMI was the issue. Crazy thing was it worked with a very long cable?? under 6 feet wouldn’t work, just black screen. as I said we ended up swapping for 4K sammys with a DP connection.
HTH
GL 
PJ