Get list of enemies in range

Hello, I am creating simple Tower Defense game and I am facing problem with getting enemies in tower range. I can't figure out how to wrote tower script to check enemies in range of sphere. I want to do that without rigidbody on enemies cus if I add it to them they are bouncing and won't use navmesh correctly. Can someone tell me how I can solve it correctly? I tried with ontriggerenter/exit but it seems I am doing something wrong cus no collisions is detected.