iPhone 7 unable to restore Error 4013

I bought this phone from a private seller. He told me that the iphone started to continiously reboot after an upgrade ( I don't exactly know which version and don't know if this is true ).

When the phone came to me I realized that it turned on, stayed on for 3- 4 second showing the Apple logo and rebooted.

I plugged the phone in, trying to restore using the DFU mode. ITunes prompted a 4013 error . After that prompt, I succesfully performed an upgrade to the latest version ( 11.2.6 ) , without any error. When ITunes ended, the phone restarted, but the boot loop problem persisted.

Then I succesessfully flashed the phone using 3uTools.

After all these steps, there's no boot loop anymore. It just stops during the boot, asking an Itunes restoring. Every attempt to restore/upgrade using Itunes fails and all I get is a 4013 error. 3uTools isn't working anymore, since, when it tries to prepare the Iphone rebooting it, the phone shows the Itunes image without responding anymore.

The seller told me that the phone has never been opened and no parts in it has never been replaced. I wonder: Can this be true? According to what I see from outside, I'd say yes, but I'll never be scure until I open it.

What do you think the problem is?

What are the next steps to take? Could you please suggest a procedure to follow in order to isolate the problem and understand where it is?

Could the battery cause this?