The Radar Range is a new weapon in Infinite. It's found in the Housewares chapter after taking the tram. Through normal story progression, you will eventually enter a shop called The Bistro. When you're in The Bistro, part of your objective is in the basements kitchen area. It's also where you will find Audio Log #14: The Kiss.
Here, you'll find a code locked door. Enter the code 3958 to unlock the door and collect the Radar Range inside.
A group of 3 enemies will spawn behind you immediately after getting it. Aim at an enemy and hold down until the enemy turns completely red and explodes, damaging the other 2 enemies in the blast radius. This will count as 1 kill towards 'Cook and Serve' and 2 towards 'Chain Reaction'.
From here, you can continue to play normally, and use it whenever you fight groups of enemies until you unlock it. You can also reload the checkpoint and repeat on the same group of enemies until they both unlock.