This is one of the responses I received:
"There were issues with the card - especially with classic Mac Pro Tower users and Sonnet pulled the current stock pending a re-work last month (late Sept. 2018). They now have new cards with a serial number beginning with 'B' (so they are Rev. B). This is information from Sonnet Support. I hope to see more positive reviews coming in pending their re-work was successful."
Sonnet shipped a replacement card today, it indeed has a "B" serial as @Auggie reported. Card worked instantly and recognized 10G from router.