What's wrong with my iphone and how to check?

Question

iPhone

The iphone was hacked - purposefully: it was not possible to log out of the account, reset to factory settings, it was constantly warming up, not being updated. In the service center, it turned out that ios was replaced by a test one (for sure not by me and definitely the device was not in the wrong hands, but connected to a third-party PC).
After flashing, the problem did not disappear: even before creating an icloud account, it gets warm at the settings stage, when you turn off the screen does not immediately go out, a blue small scroll cursor appears for a split second, just screen flashes appear. What is it and how to get rid of it, if anyone knows or there are versions, write pliz. It's not even possible to sell it normally, it's not clear what's wrong with him. I wanted to take it for examination, they asked for the cost of a new iphone of the latest model. ‍

Answer 1

In dfu mode and flash from scratch. In theory, everything will be reset to default
. Well, in the settings, then cancel the signature on the “test versions” of iOS