Why is my phone turning on and off randomly after battery change?

I have a Samsung Galaxy S7. I bought a replacement battery from ifixit almost one year ago. Last week, my university IT department changed the battery for me. I believe I calibrated the new battery. Now, the phone will turn off randomly. I will try to turn it back on, only for it to stall and turn off again. The battery also drains even quicker, with occasional drains of 20-30% instantly. The phone is barely usable.

Is this problem with the phone itself, new battery, or how it was installed? What should I do?