My suggestion is to heat the base band chip remove it, have it cleaned on the board re-ball the chip and solder back and it will work, or short cut, just heat the chip and apply liquid flux applying liberally. advise which of the methods worked.
I was perusing the comments all legitimate, but to downgrade to lower IOS from say 10.** is impossible. I personally tried it no way "Jose", As for the Icloud story well I bricked a iphone 5s after after I updated and the owner apparently had it removed from Icloud and I witness him doing it but when you put the previous owner's credentials on the phone "this user is not authorised to unlock" .
The best bet is to reflow with hot air gun, the chip may have a dry joint , with flux and check that the components adjacent to the chip become shiny almost melting, then cool it, without the metal shield attached to the frame and test, if you get a signal then you do not have to change the chip. It is quite cumbersome procedure to replace the chip but most of the time just reflowing does the trick. Mushtaq
The phone was not dropped (iphone 5S) but would not boot after a hard reset connected to iTunes and did complete restore but no display, changed LCD screen no display , the rear camera gets warm replaced battery but to no avail. No display please help. Mushtaq