DFU (Device Firmware Upgrade) mode is regarded as the ultimate flash state that can put your iOS device into where it can interface with iTunes but does not load the iBoot (bootloader on iOS devices). It allows all devices to be restored from any state. DFU mode can help solve many ios problems and it is really easy to operate.

If you want to enter DFU mode, this post is worth reading:

Hope it will be helpful for you!