Stuck in a loop after update attempt

Hi. I am working on an iphone 4S that refuses to restore and is stuck in a loop.

I have changed the battery and the charging port but to no avail. I have tried various 3rd party programs to try and reset it, but to no avail.

Phone is recognised by iTunes and the battery appears to be charging. Keep getting error 21 and 4005 during the restore process. The process crashes at this point.

There is nothing of importance on the phone and I just want to get it back to factory defaults. I took it to a repair shop and they were unsuccessful. They suggested that the fault may be caused by a faulty Baseband chip. (However they don't do chip changes) Does this sound plausible?

I don't have the expertise yet to change this chip, so if I have to send it to be changed I would like some second opinions as to whether the baseband chip idea sounds plausible or could it be something else?

Regards, Martin