Bandwidth shouldn’t be limited to HDMI 2.0 rates through a DP 1.4 port on your 5700 XT or there is other software/hardware issues elsewhere with your card.
I have the same display, although using a GTX 1660. If you are getting a black screen when trying to change to 4k @ 120hz, try unplugging/replugging the adapter and even if the resolution/refresh rate shows its already selected, try selecting it again as the adapter can be finicky and tries to read the display mode before waiting for the user to confirm the change.
CAC-1085 not working with AMD RADEON VII
I have similar setup almost.. Radeon VII and a LG CX OLED. This only shows the panel can output 60Hz. I am using a Certified Cable Zeskit 8K Ultra HD High Speed 48Gpbs HDMI -Anyone get this working with a Radeon card?? I have latest drivers from AMD, Newest Firmware for LG CX.. Supports Freesync and Gsync. High Color UHD depth turned on. Port is certified for 120hz. Tried multiple Display Ports on my card.. Same thing never will do above 8 bit and 60 Hz