Do I need a dac

Not really! The DAC in the Lumin is high quality! Using an external DAC is likely to be a sideways move! 

Great thank you would a roon improve it 

Roon will not improve your wallet. Some report that it comparatively hurts the sound quality. I didn’t notice that, but it is not for me.  I have never read that it improves ANY SQ.

An external DAC could be worse or better depending on your taste, the resolving power of your system and spending ability.  Most likely diminishing returns  make it too expensive for most.

These guys above are absolutely correct

Only if the DAC you buy is better than your built in DAC. I'm streaming from a Node 2i and was using my Parasound's DAC. I bought a used Aqua La Voce S2. It made a tremendous difference.