No worries

Generally the HD621 works well with Roon / PC.
For the first time I recommend booting the pc with the HD621 attached and on the input assigned to PC.
On PC, it should appear in the PC display and Audio settings. I set the HD621 to 5.1 in the PC audio settings and in the sound control panel in the properties for HD621 on the advanced tab. I set Exclusive mode enabled for both options.
In Roon the HD621 should be the second HD audio driver for Display Audio.
You will probably find that when you PC has a Graphics update you will have to reassigned the HD621 in Roon as endpoint.