4k @ 30Hz will be a card restriction, since there's not open settings for upto 50Hz.
As far as the 1080p goes, DP1.2 is good for 144Hz, but that's not always an option, depending on the monitor settings. It's not unheard of for monitors to clamp down on frequency with certain settings unless in 'game mode' with some options disabled like HDR or other such nonsense.
XV273K User Guide English
Standard timing table
• HDMI1 (2.0): Support max timing resolution (3840x2160-60Hz), Support HDR.
• HDMI2 (2.0): Support max timing resolution (3840x2160-60Hz), Support HDR.
• DP1: Support max timing resolution (3840x2160-120Hz), Support (FreeSync & HDR).
• DP2: Support max timing resolution (3840x2160-120Hz), Support (FreeSync & HDR).
• DP1 + DP2: AMD Gfx card can support max Timing resolution (3840x2160-144Hz), Not
Support (FreeSync & HDR).
• Need to use VESA certified DP cables
1080p@60Hz is max he can run for 1080p
4k 60 or higher will need dp 1.4
4k 144 will need RX470 as minimum GPU
since his GPU doesnt support 4k@60 on DP1.2 then it doesnt run with HBR or clock is limited to 400MHz, either way GPU upgrade is advised