Iphone 8 shut down since I upgraded it to IOS 14.5

My Iphone 8 since I upgraded it to IOS 14.5 has stopped working.

The phone constantly turns off, whether in the middle of an app or phone call. It turns off for no apparent reason and then, when it returns from the deads, the battery appears with different values.

The cell phone that worked perfectly suddenly became obsolete.

I already restarted it, I already did a total reset, I tried everything.

It continues with the same problems, if I open an app it shuts down.

The mobile phone is an important work tool for me and at the moment, the very expensive investment I made is useless, not even for making calls.

Does anyone have any idea how to solve this?