Ah, I'm sorry but I was so wrong about this one

There is indeed a model number in dxdiag log, I never noticed this!
Well you monitor is capped to 30hz
And currently you are using this mode:
I think you should really increase your resolution to native monitor mode.
Likely reason why 30hz only is because you're limited by HDMI
The link to monitor you quoted says "Upgraded HDMI connectivity supports 4K video at 60Hz" but
it says nothing about HDMI version, however 4K at 60hz must be v2.0
GPU also supports 4K @60hz
Radeon™ RX 5500 XT | Gaming Graphics | AMD
it says nothing about HDMI version but that must be v2.0 or grater (
likely v2.0b)
I'm not really sure why you don't get 60hz since both components support 4K 60hz and the site you linked says you got HDMI cable for free, so please verify that HDMI cable can handle at least v2.0
Next step is to verify GPU setting and make sure it's set to 60hz as explained here (scroll down):
Adjust Display Brightness, Resolution and Refresh Rate | AMD
If this doesn't work I would definitely consider to make use of Display Port, both the monitor and GPU supports it, GPU: DP 1.4 (but it will be capped at 1.2 due to monitor limitation) but not a problem since 1.2 supports 4k @60hz
That might work but you'll have to buy DP cable.
Btw. current 30hz limitation means that FPS is also limited to 30, even if GPU outputs higher than that.