hi my iphone 4s will only turn on when plugged into wall charger or computer. as if the battery wasnt working at all. its stuck on the connect to itunes screen. ive tried to restore it in recovery mode and in dfu mode. but keep getting error 21. ive tried all the usual fixes like the host files etc. but i am convinced its the battery. so i ordered a new battery and still the same problem. ive also checked the battery connector and there is no signs of any damage. is there any thing else i could try?