You meant now water stacked there and can't go out (overflow) at the Kitchen zink?
Toilet bowl at Kitchen also stacked?

Have you try using the "Pump" that given air pressure into the pipe or get a small flexi alumimin tube to try out?

Perhpas Indah Water can help?
But it mean need to break you KC tile and solve it? But under ground have "U" thing?
Because when my contractor doing the KC piping, it didn't have any "U" at all. It direct out to the hole inside the house. Have you open the Hole to check before?