Combine potatoes, celery, onion, ham, garlic and thyme and broth in Instant Pot.
Shut and lock lid and set to pressure for 10 minutes.
When the timer goes off, quick release steam.
While the IP is doing its magic, in separate saucepan, melt butter over medium-low heat. Whisk in flour and cook stirring constantly until thick.
Slowly stir in milk as not to allow lumps to form and all milk is added. Turn up to medium heat. Continue to stir until thick (4-5 minutes).
Add in cheese and stir until melted.
Stir the cheese mixture into the Instant Pot after you performed step 3. Set to saute on high and cook until heated through about 3 minutes. (I also will break up some of the potatoes for thickness. It gets thicker as it cools and next day).