For some reason I was struggling to enter DFU mode on my iPhone 6s, I found a useful tips that solved the problem, well.. there are two things MUST check before even go further to be able to enter DFU mode;
If you try but you couldn't enter DFU mode then:
1) You MUST plug in your iDevice to the Computer before attempt entering DFU mode, otherwise you can't, if you could then this is not for you.
2) Make sure you are using Normal USB cable instead of TYPE-C cable, for some reason USB TYPE-C cable is not supported/blocked the process for entering DFU mode. (Apple always playing around with customers).
However, since newer Mac users don't have Normal USB port, then you have to get an adapter which converts Normal USB cable > into adapter to > USB TYPE-C port.
The adapter suppose to be > Female "Normal USB" & Male "USB TYPE-C" to be able to plug it into the computer that doesn't have port for Normal USB.