After reading a comment in this thread - https://www.reddit.com/r/Netgate/comments/1bzsv4m/the_sfp_10gbaset_80m_copper_rj45_transceiver_for/ - I found the problem.
I was testing to a 1gig port on a Cisco switch. Temporarily moved it to a 10G port on a server and the interface is up and working.