I just received my Sonoff RF (https://www.itead.cc/sonoff-rf.html) and I am trying to get it to work, but I am failing miserably

Just to make sure: this is about the Sonoff RF version, not the regular Switch. My assumption is that because this Sonoff support 433mhz and PT2262 and Lighting4 it is not needed to flash the Sonoff RF: Is this correct?
I have the following setup: Raspberry Pi 3, running Domoticz V3.8153 and of course a working Rfxcom (the 433E). I am already using it with several PIR detectors and KAKU devices. However I can't get it to work with the Sonoff RF. I am hoping you can help.
The Sonoff is added to my wifi network with the EWelink app and it is working through the app. So at least the Sonoff works. My next step was to add it to Domoticz. I went into the Switches tab and clicked 'Learn light/switch'. Domoticz goes into learning mode and I try all sort of things but nothing happens. I push the button on the Sonoff, I hold it for 5-10 seconds, I switch the Sonoff on/off via the app. Nothing.
Then I tried to add it manually, but I have simply no idea what to choose in the options. Can you help me figure out what I need to do?
In Domoticz > Setup > Hardware I made sure the RFXcom accepts Lighting4.
All the help is appreciated. Thanks a lot!