In Windows 10 Sound Panel under "Default Audio Device" when you press "Configure" when you mouse click on the icon, what does it show? The TV as being Stereo, Correct? You don't see your Soundbar in the Sound Panel,Correct?
If your HDMI is connected to the TV directly that is why you are only seeing Stereo since the TV only has two speakers.
What you need to do is connect the HDMI cable from the GPU Card HDMI Output to the Soundbar HDMI Input. Then from the Soundbar HMDI Output to your Television HDMI Input.
This is the only way that am aware of that Windows 10 will recognize your 5.1 Soundbar. Make sure you have the latest Soundbar driver installed if applicable.