I'm trying to connect my 2015 27" Retina iMac to an LG 27UK850 monitor as a second screen using a mini display port to HDMI cable. It seems to recognize the 2nd screen, and both the external monitor and macOS acts as if it's connected (I can move the mouse cursor and put windows there, plus it shows up on System Preferences -> Displays -> Arrangement), but the screen is blank. I used a USB-c cable to connect my 2016 Macbook Pro to verify that the monitor does indeed work. According to the manual, the monitor should be compatible with this iMac model, and I've installed the relevant software.
The next thing I'm thinking about trying is a different cable (this one is new, could be faulty?) or perhaps get a mini DP to DP cable, or a mini DP to HDMI adapter to use the HDMI-HDMI cable that came with the monitor. Is there something obvious I'm missing here? I figured I should just be able to plug it in and it should just work, as it did with my Macbook Pro.
The next thing I'm thinking about trying is a different cable (this one is new, could be faulty?) or perhaps get a mini DP to DP cable, or a mini DP to HDMI adapter to use the HDMI-HDMI cable that came with the monitor. Is there something obvious I'm missing here? I figured I should just be able to plug it in and it should just work, as it did with my Macbook Pro.