There’s a few ways to do this, I think the simplest, if you have the PCIE slot room would be to buy an ethernet card that supports 10/5/2.5/1GbE network speed negotiation.
But since I only have one real usable slot in my pfSense machine, and I need 10G for LAN, I opted for a dual-port Mellanox ConnectX-3 SFP card that I purchased off of eBay for $21 USD. An incredible buy. I would definitely recommend putting a fan on the heatsink if you have room, or blowing across the card is even better to help cool the SFP modules themselves. Especially when using ethernet modules, they run hotter than fiber modules.
For the SFP module, I chose a H!Fiber branded module: https://www.amazon.com/dp/B06XQBFHNL
Per the previous guide, I updated my /conf/dhcp6_att.conf to replace interface re0 with interface mlxen1 (by process of elimination or checking ifconfig to determine which slot is which. What the bracket of the card calls port 0 actually was my mlxen1 interface, in this case.
In pfSense, you will see it negotiate as 10Gbase-CX4. The speed of the port does not matter, assuming it supports your standard 1G/10G SFP+ port speeds. The module is what matters. The modules SFP end will interface with the port at 10Gbase-CX4, and the ethernet side of the module will interface with your modem at 5Gbase-T. You’ll have to login to your AT&T modem to confirm the speed of the client (under Device > Device List). Make sure you’re connected to the 5G port on the back of your AT&T BGW320!
As I only pay for the 1G plan, I am at least now able to pull the full speed.
Insanity.