iphone 6s wont power on

I have an iphone 6s. Last week (Wednesday 9/12/19) it refused to charge. Got stuck on the screen with the battery. I did a hard reset. No better. I connected it to iTunes and did a hard reset. iTunes offered to restore and update. iTunes started the process and managed to do the restore to factory settings. iTunes started the update. About 15 minutes in, iTunes tells me "the phone cannot be found." This process was repeated many, many, many times. After two days of playing with this process, success. The phone had the apple logo and was charged to 100% on Saturday 9/14/19.

In the meantime, I went to ATT and bought an iphone 7 because I needed a phone, and figured the 7 can go to my teenager once I resolve the 6s issue.

Yesterday I took the phones to the ATT store, transferred my number back to the 6s, got my teenager set up on the 7. Phones came home and worked great. At bedtime, my 6s was plugged in, charging, showed 78% of battery power. By 5:15 am it was dead even though it was plugged in (my alarm didn't go off!)

I'm back to stuck on the battery screen. I've tried the reset again (power + home button) and can't even get the apple logo to appear after holding them for up to a minute. I've connected it to iTunes on two different computers and iTunes won't recognize it. I've connected it to iTunes and attempted the reset (power + home button) and the DFU (home button until iTunes recognizes the device). iTunes won't recognize it.

I've cleaned the charging port, tried different cables, different outlets, apple-only cords and chargers, iTunes resets, everything I can think of.

I believe this is a software issue since I had it up and running just fine yesterday. Is there a way to force a software update if I can't get iTunes to recognize it?