I found Iperios and it is a modded server where players can make their own nation. You can pick your own kind of government and set laws and ranks for your group.
If you do not want to run a nation you can join one that already exists. It is built around the All the Mods 10 modpack and it says to use version 5.5.
To get in you have to join their Discord and ask for whitelist. After you are accepted a staff member helps you start and gives a beginner kit so you can get going.
WebsiteDiscord