Question about battery replacement and problem that followed.

Try the following before doing anything:

1. QUICKLY press and release volume up button
2. Immediately press and QUICKLY release volume down button
3. Immediately press and KEEP HOLDING the side power button. KEEP HOLDING it no matter what you see on the screen until you see the Apple Logo, then let go. This part can take up to 30 seconds, so KEEP HOLDING

If this doesn't work, I'd take it back to Apple as they replaced the battery and it would have a 90 day warranty. I would have honestly been very disappointed to receive a replacement phone with 81% Battery Health to be honest. That phone was not refurbished well to begin with and it should have at least already had a new battery in it. If you want to take that up with T-Mobile, that's between you and them as Apple wouldn't be involved there. You should have actually taken that up with T-Mobile when you got the replacement phone. Now that you've paid to have the battery replaced, they will unlikely be willing to do anything for you.

Not sure of the terms of the Assurant/TMobile insurance, but replacing a device with one that has 81% battery health is certainly not a good program. The battery is expected to need replacing once it falls below 80%.

To determine exactly what is going on with the phone, it will take a trip to the Apple Store and the battery that they installed will still be covered by their repair warranty. They will give you cost of repair options for any other condition. Unfortunately due to your previous experience with that insurance program, it is difficult to recommend that as a solution, but at least you will have more information from Apple to make that determination.

The Force Restart provided by lobsterghost1 is a great first step. Also make sure the issue is not with the charger itself or the cable if you have another one to test. That is something that Apple can also do if you do not have one.