Before using RVO, I was using OnTriggerStay to make my AI start to attack the enemy, but now with RVO, I cannot use it, because they don't collide.
Have any way to detect if they are really close enought to start to attack?
You can attach triggers to the agent, that should work fine. Just make sure to also attach a kinematic rigidbody to ensure the collision events are also sent.
Otherwise a simple loop over all your agents to check their distances should work well too.