I think I can put my finger on this, Kassee ........... as it were ;o)
It sounds as though your drain runs are clear if you've had a look in the manholes etc. You'll find the blockage is just a few inches from the WC pan. Most pans have a 90 degree pan connector on them.............. then, quite often another 90 degree bend to connect to a soil stack. It's those tight initial bends where a blockage usually builds up after flushing with a cistern that isn't working properly.
Too much information Alert!!!
A flushing mechanism is quite low-tech, but also critical in the way it clears a pan with a small amount of water.
The first thing I would try would be Methyl's "big plunger". I know I've said this before, but it's like the rubber thing on the end of a Dalek's arm ;o)
Shove it right into the bottom of the pan and pull with some gusto. In other words .............. give it some ...........