I have the Intel X520-DA2 (dell branded) on my desktop and router
I then use this adapter https://www.amazon.com/gp/product/B07QXNQTXG to connect to the 5Gbit port, AT&T shows it's at 5Gbe, Pfsense shows 10Gbe. Could be a bug, but I'm able to speedtest at my provisioned speeds without issue (2Gbit).
Got it working for $45 (already had the dual SFP+ card). https://www.speedtest.net/result/11113609420.png
Using a dual 10gig SFP+ X520 with a RJ45 SFP+ transceiver that can negotiate to 2.5GHz. https://www.amazon.com/gp/product/B07QXNQTXG
OP reporting back.
Got it working for $45 (already had the dual SFP+ card). https://www.speedtest.net/result/11113609420.png
Using a dual 10gig SFP+ X520 with a RJ45 SFP+ transceiver that can negotiate to 2.5GHz. https://www.amazon.com/gp/product/B07QXNQTXG