Combine water, beef broth, and beer in your Instant Pot or pressure cooker.
Stir in the garlic.
Place the trivet inside the instant pot insert.
Sprinkle and rub all sides of the brisket with brown sugar.
Place the beef brisket on the trivet and sprinkle spice packet on top.
Close and lock the lid.
Select HIGH PRESSURE; set timer for 80 minutes.
Release pressure carefully using the quick-release for about 5 minutes.
Unlock and remove the lid.
Transfer brisket to a cutting board; cover with aluminum foil and let rest for 15 minutes before cutting.
In the meantime, remove the trivet from the pot.
Place potatoes, carrots, onions, and cabbage in the bottom of the pressure cooker. Close and lock the lid.
Select HIGH PRESSURE and set the timer for 5 minutes.
Release pressure using the quick-release method for about 5 minutes.
Unlock and remove the lid.
Drain and remove vegetables from the instant pot; taste for salt and pepper, and adjust accordingly.
Thinly slice the corned beef and serve with the prepared veggies.