Measure out frozen corn and set aside to thaw as you prepare the stew.
Turn on the sauté function of your Instant Pot or another electric pressure cooker. (These instructions will refer to Instant Pot settings.) Add onion and celery with about a tablespoon of water. Cover and cook until soft, stirring occasionally, about 5 minutes. Add more water if needed to prevent sticking. Add garlic and cook until it softens, another 2-3 minutes.
Add all remaining ingredients through ground black pepper. Turn off sauté function on Instant Pot. Add pressure cooking lid and seal making sure pressure valve is on sealing and not venting. Cook at high pressure for 12 minutes and let pressure release naturally.
When pressure indicator has dropped, open lid and smash most of the softened sweet potato with a fork. You want to create a thickened sauce. Now stir in the corn, stir, cover and let sit for 2-3 minutes.
Stir, salt to taste, and sprinkle each serving with cilantro, if using. It's a nice touch but totally not required.