Use both manual switch and WiFi Smarth switches together
The Wifi switch you mention is not meant to work with a switch. With only an L and N terminal it will connect or break the line to turn on the light. If you install a switch, the switch will turn the light off if it's on but it will kill power to the Wifi switch as well. You would have to turn the switch back on to use the Wifi. Also if you wanted to turn the light on the switch would not be able to do that. 

To do what you want, you need a 3way or 4way Wifi switch.