The Modrons have the following trait:
Axiomatic Mind. The [modron] can’t be compelled to act in a manner contrary to its nature or its instructions.
What does this mean?
It sounds to me like nonphysical means fail to make it act in a contrary manner. It is pretty clear that persuasion and intimidation would never work in such a case and since I, as GM, decide on skill checks anyway, that is no problem.
But, does this mean that the modron is immune against spells such as suggestion? Or against the charmed and frightened conditions when they would ask for contrary actions?
Clarification: "spells like suggestion" means spells that can be used to force a course of action, also including mass suggestion, command and dominate monster.