that Dell display appears to be a 5120 by 2160 display with
HDMI that claims the ability to support 5K at 120 Hz
ThunderBolt Port that seems to support display data as well as ThunderBolt data downstream
DisplayPort 1.4 with ability to support 5K at 120 Hz
• DIRECT connect with a certified ULTRA HDMI cable (adapters Prohibited) [longer than one meter may not work] or
• Thunderbolt cable shorter than 0.8 meters for Apple cables, 0.5 meters for brand-X cables or
• DisplayPort adapter/cable specified for these data rates AND the adapter electronics are within ONE meter of the computer-end AND the cable beyond is shorter than one additional meter.
... then it should work.
These data rates are VERY fast and unrelenting. Use of Docks or additional adapters is NOT likely to work.