Thanks all

The directions i followed said to go to my.hdhomerun.com but the url is slightly different and device registration was my issue.

Purchased a refurbed flex 4k direct from SiliconDust
Purchased yearly EPG / XML access
Retrieved device auth code by going to discover page
appended auth code to this url: https://api.hdhomerun.com/api/xmltv?DeviceAuth=

Got 403 Forbidden

I visited via browser & attempted to use the url in my plex ...