Combine the pasta, broth, butter, salt, pepper and garlic in the Instant Pot and twist on the locking lid.
Select the Manual setting. Set the Instant Pot for ½ of the time listed on the package of pasta.
When the pot has finished the pressurized cook time and beeps, allow everything to Natural Pressure Release for 2 minutes. Then cover the steam valve with a clean, damp cloth. This will help release the steam without dangerous and messy liquids spraying everywhere.
Give the pasta a solid stir and then add in the milk and cheeses.
Stir everything together until smooth and creamy. Serve right away.