Preheat the oven to 350 degrees F (175 degrees C). Line a baking pan with aluminum foil.
Place chicken carcass, onions, carrots, celery, and garlic on the prepared baking pan.
Bake in the preheated oven until carcass is browned, about 1 hour.
Remove pan from oven; scrape carcass and roasted vegetables into an electric pressure cooker (such as Instant Pot(R)). Add tomato, peppercorns, cider vinegar, and salt. Fill pot 2/3 full with water; close and lock the lid. Select the Soup setting according to manufacturer's instructions and set timer for 30 minutes. Allow 10 to 15 minutes for pressure to build.
Release pressure using the natural-release method according to manufacturer's instructions, 10 to 40 minutes.
Strain broth through a layer of cheesecloth into another container. Let cool, at least 20 minutes. Cover and refrigerate until a layer of fat solidifies on the surface, 4 hours to overnight. Remove and discard the fat.