The iPhone 6 and older models were easy to force restart, all you had to do is hold the power and home button down until the apple logo appeared. That changed with the iPhone 7 due to the way the home button works. The following website will show you all you need to know about how to force restart, enter recovery mode, or enter DFU mode.
iPhone 7: How to force restart, enter Recovery Mode, and enter DFU mode
Since the Home button on iPhone 7 isn’t a mechanical button anymore, and is instead a capacitive solid-state button controlled by software, it’s no surprise that performing hard resets, entering recovery, and entering into DFU mode no longer works the same way on iPhone 7.