I ended up solving the problem by using an Elcom BSR-140 relais that I’m controlling via an arduino. Maybe the opener could also be hooked up to the BSR-140; did not investigate that, as the arduino works for me 
For me it works without someone having to ring. But that seems to depend on the settings of the door relais; according to the elcom docs there is a jumper that enables this. No way to know how your door is configured without trying…