This is a short guide on how to change the AIs sight distance, this controls how far the AI can see. Start by going to the BP_MasterAIController then in the components tab select the AIPerception. Then click the Senses Config drop down option then the 0 drop down option then the Sense Drop down option and you should see the options shown below.

Sight Radius is the distance the AI can see in CM so 5000 would be 50 meters. You can increase or decrease this value as you need. You will also need to change the Loss Sight Radius value as well I recommend just keeping it 500 more then your Sight Radius.

DO NOT use very low values anything under 200 as that is only 2 meters in distance.

Finally in the BP_MasterAIController in the Class Defaults change the Starting Sight Radius to the same value you set the Sight Radius as.

Once this is done your AI will now be using you new Sight Distance.

If you want to know how to change the Hearing Distances check out the guide linked below.