For others that come here from search engines :
Here , there is shown how to transform SONOFF RFR3 Dry Contact
Credits goes to Vincenzo Caputo :
https://www.youtube.com/channel/UC-_QPzJhqFr1p5oNljRPRDA
There are 2 cuts to be made, in red (see video), and one bridge (in yellow) to be soldered.
Tested and working, dry contact with SONOFF, easy to undone if necessary, just desolder the wire and make bridge where you cut.