a normal rj12 should work. if you have a multimeter you can check all 6 pins of the cable. does your doser have a display on the top?
and i still think you need to use the option with the digital power bar connektion.
try to set the switch channels as always on. so you can see, there is a problem with the connectiion doser an p3 or with the configuration of the dosing pumps.