I found a modded server that runs on All the Mods 10 ATM10 version 5.5 and the whole vibe is nations. You can make your own nation and pick how it is run with your own government laws and ranks.
If you do not want to start from zero you can join a nation that already exists. To get in you join the Discord and ask to be whitelisted then staff helps you get started and gives you a beginner kit.
Make sure you have the ATM10 modpack installed on version 5.5 before you try to play.
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