That stuff is a waste of time and money IMO. You've already got a snake 7m along the pipe, if that 7m is full of water, the chemical has a long way to go, and will be very dilute, by the time it gets to where its needed.
Can you see the connection where the waste pipe goes into the floor, i.e. does it go into a bigger pipe at floor level?
Can you see the connection where the waste pipe goes into the floor, i.e. does it go into a bigger pipe at floor level?