I would treat axiomatic mind as almost immunity to charmed, frightened, and other mind altering spells or conditions. They can't be compelled to act in a manner other than their nature or instructions. If they instructions are to attack the PCs and the wizard casts charm person, I'd have the Modron roll a save. If it fails the save, and the PC tells it to do anything other than attack the PCs, that doesn't work. In the same combat if the warlock makes a Modron frightened, the Modron might go after a different target first, but if the warlock and the frightened Modron are the last two standing, the Modron will advance toward the warlock. It can't be compelled to act in a manner contrary to its instructions.