Hi JamesAdams,
Have you try to disable NIC from the bios and connect locally to launch powertop?
This is how I found my Intel 2.5G was preventing any c6-c8 state.
If it's the case with your Realtek 2.5G you can try to force gigabit speed instead of 2.5G (where eth0 is your network card name)
ethtool -s eth0 speed 1000 duplex full autoneg on